MCC usually presents as a firm, painless, nodule (up to 2 cm diameter) or mass (>2 cm diameter). These flesh-colored, red, or blue tumors typically vary in size from 0.5 cm (less than one-quarter of an inch) to more than 5 cm (2 inches) in diameter, and usually enlarge rapidly. Although MCC's may arise almost anywhere on the body, about half originate on sun-exposed areas of the head and neck, one-third on the legs, and about one-sixth on the arms. In about 12% of cases, no obvious anatomical site of origin ("primary site") can be identified.

Merkel-cell cancers tend to invade locally, infiltrating the underlying subcutaneous fat, fascia, and muscle, and typically metastasize early in their natural history, most often to the regional lymph nodes. MCCs also spread aggressively through the blood vessels, particularly to liver, lung, brain, and bone.

Early signs of melanoma are changes to the shape or color of existing moles or, in the case of nodular melanoma, the appearance of a new lump anywhere on the skin. At later stages, the mole may itch, ulcerate or bleed. Early signs of melanoma are summarized by the mnemonic "ABCDE":

- Asymmetry

- Borders (irregular with edges and corners)

- Color (variegated)

- Diameter (greater than , about the size of a pencil eraser)

- Evolving over time

These classifications do not, however, apply to the most dangerous form of melanoma, nodular melanoma, which has its own classifications:

- Elevated above the skin surface

- Firm to the touch

- Growing

Metastatic melanoma may cause nonspecific paraneoplastic symptoms, including loss of appetite, nausea, vomiting and fatigue. Metastasis of early melanoma is possible, but relatively rare: less than a fifth of melanomas diagnosed early become metastatic. Brain metastases are particularly common in patients with metastatic melanoma. It can also spread to the liver, bones, abdomen or distant lymph nodes.

It presents as a slow growing mass that especially affects tendons and aponeuroses and it is deeply situated. Patients often perceive it as a lump or hard mass. It causes either pain or tenderness but only until it becomes large enough. This kind of tumor is commonly found in the extremities especially around the knee, feet and ankle. Patients diagnosed with clear cell sarcoma are usually between the ages of 20 and 40.

An invasive tumor arising from a classical lentigo maligna. Usually a darkly pigmented raised papule or nodule, arising from a patch of irregularly pigmented flat brown to dark brown lesion of sun exposed skin of the face or arms in an elderly patient.

Typical signs of acral lentiginous melanoma include the following

- Longitudinal tan, black, or brown streak on a finger

- Pigmentation of proximal nail fold

- Areas of dark pigmentation (on palms of hands)

Warning signs are new areas of pigmentation, or existing pigmentation that shows change. If caught early, acral lentiginous melanoma has a similar cure rate as the other types of superficial spreading melanoma.

Merkel-cell carcinoma (MCC) is a rare and highly aggressive skin cancer, which, in most cases, is caused by the Merkel cell polyomavirus (MCV) discovered by scientists at the University of Pittsburgh in 2008. It is also known as cutaneous APUDoma, primary neuroendocrine carcinoma of the skin, primary small cell carcinoma of the skin, and trabecular carcinoma of the skin.

Approximately 80% of Merkel-cell carcinomas are caused by MCV. The virus is clonally integrated into the cancerous Merkel cells. In addition, the virus has a particular mutation only when found in cancer cells, but not when it is detected in healthy skin cells. Direct evidence for this oncogenetic mechanism comes from research showing that inhibition of production of MCV proteins causes MCV-infected Merkel carcinoma cells to die but has no effect on malignant Merkel cells that are not infected with this virus. MCV-uninfected tumors, which account for approximately 20% of Merkel-cell carcinomas, appear to have a separate and as-yet unknown cause. These tumors tend to have extremely high genome mutation rates, due to ultraviolet light exposure, whereas MCV-infected Merkel cell carcinomas have low rates of genome mutation. No other cancers have been confirmed so far to be caused by this virus. Because of the viral origin for this cancer, immunotherapies are a promising avenue for research to treat virus-positive Merkel-cell carcinoma.

This cancer is considered to be a form of neuroendocrine tumor. While patients with a small tumor (less than 2 cm) that has not yet metastasized to regional lymph nodes have an expected 5-year survival rate of more than 80 percent, once a lesion has metastasized regionally, the rate drops to about 50 percent. Up to half of patients that have been seemingly treated successfully (i.e. that initially appear cancer-free) subsequently suffer a recurrence of their disease. Recent reviews cite an overall 5-year survival rate of about 60% for all MCC combined.

Merkel-cell carcinoma occurs most often on the sun-exposed face, head, and neck.

Melanoma, also known as malignant melanoma, is a type of cancer that develops from the pigment-containing cells known as melanocytes. Melanomas typically occur in the skin, but may rarely occur in the mouth, intestines, or eye. In women, they most commonly occur on the legs, while in men they are most common on the back. Sometimes they develop from a mole with concerning changes including an increase in size, irregular edges, change in color, itchiness, or skin breakdown.

The primary cause of melanoma is ultraviolet light (UV) exposure in those with low levels of skin pigment. The UV light may be from either the sun or from other sources, such as tanning devices. About 25% develop from moles. Those with many moles, a history of affected family members, and who have poor immune function are at greater risk. A number of rare genetic defects such as xeroderma pigmentosum also increase risk. Diagnosis is by biopsy of any concerning skin lesion.

Using sunscreen and avoiding UV light may prevent melanoma. Treatment is typically removal by surgery. In those with slightly larger cancers, nearby lymph nodes may be tested for spread. Most people are cured if spread has not occurred. For those in whom melanoma has spread, immunotherapy, biologic therapy, radiation therapy, or chemotherapy may improve survival. With treatment the five-year survival rates in the United States is 98% among those with localized disease and 17% among those in whom spread has occurred. The likelihood that it will come back or spread depends how thick the melanoma is, how fast the cells are dividing, and whether or not the overlying skin has broken down.

Melanoma is the most dangerous type of skin cancer. Globally, in 2012, it newly occurred in 232,000 people. In 2015 there were 3.1 million with active disease which resulted in 59,800 deaths. Australia and New Zealand have the highest rates of melanoma in the world. There are also high rates in Northern Europe and North America, while it is less common in Asia, Africa, and Latin America. Melanoma is more common in men than women. Melanoma has become more common since the 1960s in areas which are mostly populated with white people.

Lentigo maligna melanoma is a melanoma that has evolved from a lentigo maligna. They are usually found on chronically sun damaged skin such as the face and the forearms of the elderly. The nomenclature is very confusing to both patients and physicians alike.

Lentigo maligna is the non-invasive skin growth that some pathologists consider to be a melanoma-in-situ. A few pathologists do not consider lentigo maligna to be a melanoma at all, but a precursor to melanomas. Once a lentigo maligna becomes a lentigo maligna melanoma, it is treated as if it were an invasive melanoma.

Nodular melanoma (NM) is the most aggressive form of melanoma. It tends to grow more rapidly in thickness (penetrate the skin) than in diameter. Instead of arising from a pre-existing mole, it may appear in a spot where a lesion did not previously exist . Since NM tends to grow in depth more quickly than it does in width, and can occur in a place that did not have a previous lesion, the prognosis is often worse because it takes longer for a person to be aware of the changes. NM is most often darkly pigmented; however, some NM lesions can be light brown, multicolored or even colorless (non-pigmented). A light-colored or non-pigmented NM lesion may escape detection because the appearance is not alarming, however an ulcerated and/or bleeding lesion is common. Polypoid melanoma is a virulent variant of nodular melanoma.

The microscopic hallmarks are:

- Dome-shaped at low power

- Epidermis thin or normal

- Dermal nodule of melanocytes with a 'pushing' growth pattern

- No "radial growth phase"

Basal-cell skin cancer (BCC) usually presents as a raised, smooth, pearly bump on the sun-exposed skin of the head, neck or shoulders. Sometimes small blood vessels (called telangiectasia) can be seen within the tumor. Crusting and bleeding in the center of the tumor frequently develops. It is often mistaken for a sore that does not heal. This form of skin cancer is the least deadly and with proper treatment can be completely eliminated, often without scarring.

These tumors are usually benign, but can become malignant over time. They vary in size, and can be found as singles or multiples. They are most commonly found in mature grey horses (less than 15 years old) and are typically found under the tail, around the anus, and on the external genitalia.

When the tumor is large and there is presence of necrosis and local recurrence, the prognosis is poor. Presence of metastasis occurs in more than 50% cases and the common places of its occurrence are the bone, lymph node and lungs. Five-year survival rates, which are reported to be between 50-65%, can be misleading because the disease is prone to late metastasis or recurrence. Ten and twenty-year survival rates are 33% and 10%, respectively.

Squamous-cell skin cancer (SCC) is commonly a red, scaling, thickened patch on sun-exposed skin. Some are firm hard nodules and dome shaped like keratoacanthomas. Ulceration and bleeding may occur. When SCC is not treated, it may develop into a large mass. Squamous-cell is the second most common skin cancer. It is dangerous, but not nearly as dangerous as a melanoma.

The appearance and number of sarcoids can vary, with some horses having single or multiple lesions, usually on the head, legs, ventrum and genitalia or around a wound. The distribution pattern suggests that flies are an important factor in the formation of sarcoids. Sarcoids may resemble warts (verrucous form), small nodules (nodular form), oval hairless or scaly plaques (occult form) or very rarely, large ulcerated masses (fibroblastic form). The occult form usually presents on skin around the mouth, eyes or neck, while nodular and verrucous sarcoids are common on the groin, penile sheath or face. Fibroblastic sarcoids have a predilection for the legs, groin, eyelid and sites of previous injury. Multiple forms may also be present on an individual horse (mixed form). Histologically, sarcoids are composed of fibroblasts (collagen producing cells) that invade and proliferate within the dermis and sometimes the subcutaneous tissue but do not readily metastasize to other organs. Surgical biopsy can definitively diagnose sarcoids, but there is a significant risk of making sarcoids worse. Therefore, diagnosis based solely on clinical signs, fine-needle aspiration or complete excisional biopsy are safer choices.

Desmoplastic melanoma (also known as a "Neurotropic melanoma," or "Spindled melanoma") is a rare cutaneous condition characterized by a deeply infiltrating type of melanoma with an abundance of fibrous matrix. It usually occurs in the head and neck region of older people with sun-damaged skin. Diagnosis can be difficult as it has a similar appearance to sclerosing melanocytic nevi as well as some nonmelanocytic skin lesions such as scars, fibromas, or cysts.

Desmoplastic melanomas tend to recur locally, with distant metastasis being less common.

Acral lentiginous melanoma is a kind of lentiginous skin melanoma. Melanoma is a potentially serious skin cancer that arises from pigment cells (melanocytes). Although acral lentiginous melanoma is rare in people with lighter skin types, it is the most common subtype in people with darker skins. Acral lentiginous melanoma is observed on the palms, soles, under the nails and in the oral mucosa. It occurs on non-hair-bearing surfaces of the body, which may or may not be exposed to sunlight. It is also found on mucous membranes. It is the most common form of melanoma diagnosed amongst Asian and sub-Saharan African ethnic groups. The average age at diagnosis is between sixty and seventy years.

The most common sites for melanotic tumors are on the under-side of the tail near the base, on the prepuce, around the mouth or in the skin over the parotid gland (near the base of the ear). Tumors will initially begin as single, small raised areas that may multiply or coalesce into multi-lobed masses (a process called melanomatosis) over time. Horses under 2-years-old can be born with or acquire benign melanotic tumors (called melanocytomas), but these tumors are often located on the legs or trunk, not beneath the tail as in older animals.

An equine melanoma is a tumor that results from the abnormal growth of melanocytes in horses. Unlike human melanomas, they are not thought to be caused by exposure to ultraviolet light. Melanomas are the third most common type of skin cancer in horses, with sarcoids being the first most prevalent and squamous-cell carcinoma being second. They are typically rounded black nodules that vary in size and can usually be found underneath the dock of the tail, the anal, perianal and genital regions, perineum, lips, eyelids, and sometimes near the throatlatch.

These tumors can be either benign, meaning not cancerous, or malignant, meaning cancerous; while the benign tumors typically need little treatment to no treatment, the malignant tumors can cause serious problems and can potentially be life-threatening. Different methods are used to determine if the tumor is malignant and if it has spread to other organs. Methods used to determine malignancy include fine needle aspirate, biopsy, or complete removal. To determine if the tumor has metastasized, a rectal examination or an ultrasound can be performed; the most frequent location for metastasis include the lymph nodes, spleen, liver, the abdominal wall, the lungs, and blood vessels. If the tumor becomes large enough, it can cause weight loss or colic. It may also affect the horse’s ability to turn their head from side to side and eat and drink comfortably if the tumor is on the throat latch area, or cause faecal impaction if tumor is on the anal region. These tumors can also be a serious issue if they have metastasized or if they become large and ulcerate, bleed, then become infected.

Often, this disease evolves from a precursor lesion, usually a dysplastic nevus. Otherwise it arises in previously normal skin. A prolonged radial growth phase, where the lesion remains thin, may eventually be followed by a vertical growth phase where the lesion becomes thick and nodular. As the risk of spread varies with the thickness, early SSM is more frequently cured than late nodular melanoma.

The microscopic hallmarks are:

- Large melanocytic cells with nest formation along the dermo-epidermal junction.

- Invasion of the upper epidermis in a pagetoid fashion (discohesive single cell growth).

- The pattern of rete ridges is often effaced.

- Invasion of the dermis by atypical, pleomorphic melanocytes

- Absence of the 'maturation' typical of naevus cells

- Mitoses

Uveal melanomas, often referred to by the media and in the general population as ocular melanomas, may arise from any of the three parts of the uvea, and are sometimes referred to by their location, as choroidal melanoma, ciliary body melanoma, or iris melanoma. Large tumors often encompass multiple parts of the uvea and can be named accordingly. True iris melanomas, originating from within the iris as opposed to originating elsewhere and invading the iris, are distinct in their etiology and prognosis, such that the other tumors are often referred to collectively as Posterior uveal melanomas.

Characteristics include a blue/black stain of skin initially. Skin is thin, about 4-5 cell layers thick, which is often related to aging. Histological features include epidermal atrophy and increased number of melanocytes.

Esthesioneuroblastoma will first frequently present as a nasal mass. The most common signs and symptoms of esthesioneuroblastoma are nasal obstruction (70%) and epistaxis (50%). Less common symptoms include hyposmia (loss of smell), headache, rhinorrhea, vision loss, proptosis, facial pain, diplopia (double vision), masses in the neck and changes in mental status. Esthesioneuroblastoma occurs in the upper nasal cavity, near the optic nerves and optic chiasm. Thus, tumor growth can impinge nerve function and result in vision loss and diplopia. As the tumor metastasizes to the oral cavity, there can be tooth pain and tooth mobility.

Meningeal carcinomatosis is a condition in which a solid tumor diffusely spreads to the leptomeninges. Lung tumors, breast tumors, and malignant melanoma comprise the majority of solid tumors spreading to the leptomeninges.

The symptoms of choroid plexus carcinoma are similar to those of other brain tumors. They include:

- Persistent or new onset headaches

- Macrocephaly or bulging fontanels in infants.

- Loss of appetite (refusal to take food in infants)

- Papilledema

- Nausea and emesis

- Ataxia

- Strabismus

- Developmental delays

- Altered mental status

Superficial spreading melanoma (also known as "superficially spreading melanoma") (SSM) is usually characterized as the most common form of cutaneous melanoma in Caucasians. The average age at diagnosis is in the fifth decade, and it tends to occur on sun-exposed skin, especially on the backs of males and lower limbs of females.