Symptoms of cancer in dogs may include:

- Lumps (which are not always malignant, but should always be examined by a vet)

- Swelling

- Persistent sores

- Abnormal discharge from any part of the body

- Bad breath

- Listlessness/lethargy

- Rapid, often unexplained weight loss

- Sudden lameness

- Offensive odor

- Black, tarry stools (a symptom of ulcers, which can be caused by mast cell tumors)

- Decreased or loss of appetite

- Difficulty breathing, urinating or defecating

While each type of cancer has its own distinctive symptoms, most of them can indicate their presence by the occurrence and the prolonged presence of any common symptom. Some of the general symptoms of cancer in cats are:

- Any lump that changes shape or size

- Any sore that does not heal

- Change in bowel or bladder habits

- Difficulty eating or swallowing

- Difficulty urinating or defecating

- Unexplained bleeding or discharge from body

- Loss of appetite

- Chronic weight loss

- Coughing or difficulty breathing

- Stiffness

- Oral odor

Note that ravenous hunger, while a sign of other diseases like hyperthyroid, can also be a sign of cancer.

Lymphoma is the most common form of cancer in cats, which is often associated with feline leukemia virus, and accounts for 25 percent of all cases. Feline lymphoma usually strikes the digestive system, causing excessive vomiting and diarrhea. Other common symptoms of lymphoma in cats include swollen lymph nodes, loss of appetite, weight loss, and difficulty breathing.

Cancer is the leading cause of death in dogs. It is estimated that 1 in 3 domestic dogs will develop cancer, which is the same incidence of cancer among men. Dogs can develop a variety of cancers and most are very similar to those found in humans. Dogs can develop carcinomas of epithelial cells and organs, sarcomas of connective tissues and bones, and lymphomas or leukemias of the circulatory system. Selective breeding of dogs has led certain pure-bred breeds to be at high-risk for specific kinds of cancer.

Veterinary oncology is the medical study of cancer in animals, and can be diagnosed and treated by specialized veterinarians called veterinary oncologists.

Signs vary but may include bleeding from the nose, nasal discharge, facial deformity from bone erosion and tumor growth, sneezing, or difficulty breathing.

When cancer begins, it produces no symptoms. Signs and symptoms appear as the mass grows or ulcerates. The findings that result depend on the cancer's type and location. Few symptoms are specific. Many frequently occur in individuals who have other conditions. Cancer is a "great imitator". Thus, it is common for people diagnosed with cancer to have been treated for other diseases, which were hypothesized to be causing their symptoms.

People may become anxious or depressed post-diagnosis. The risk of suicide in people with cancer is approximately double.

Local symptoms may occur due to the mass of the tumor or its ulceration. For example, mass effects from lung cancer can block the bronchus resulting in cough or pneumonia; esophageal cancer can cause narrowing of the esophagus, making it difficult or painful to swallow; and colorectal cancer may lead to narrowing or blockages in the bowel, affecting bowel habits. Masses in breasts or testicles may produce observable lumps. Ulceration can cause bleeding that, if it occurs in the lung, will lead to coughing up blood, in the bowels to anemia or rectal bleeding, in the bladder to blood in the urine and in the uterus to vaginal bleeding. Although localized pain may occur in advanced cancer, the initial swelling is usually painless. Some cancers can cause a buildup of fluid within the chest or abdomen.

In male dogs, the tumor affects the penis and foreskin. In female dogs, it affects the vulva. Rarely, the mouth or nose are affected. The tumor often has a cauliflower-like appearance. Signs of genital TVT include a discharge from the prepuce and in some cases urinary retention, from blockage of the urethra. Signs of a nasal TVT include nasal fistulae, nosebleeds and other nasal discharge, facial swelling, and enlargement of the submandibular lymph nodes.

The most common type of cancers affecting the animal's nose are carcinomas and

sarcomas, both of which are locally invasive. The most common sites for metastasis are the lymph nodes and the lungs, but can also include other organs.

There are few early warning signs that a patient has a DSRCT. Patients are often young and healthy as the tumors grow and spread uninhibited within the abdominal cavity. These are rare tumors and symptoms are often misdiagnosed by physicians. The abdominal masses can grow to enormous size before being noticed by the patient. The tumors can be felt as hard, round masses by palpating the abdomen.

First symptoms of the disease often include abdominal distention, abdominal mass, abdominal or back pain, gastrointestinal obstruction, lack of appetite, ascites, anemia, and/or cachexia.

Other reported symptoms include unknown lumps, thyroid conditions, hormonal conditions, blood clotting, kidney or urological problems, testicle, breast, uterine, vaginal, or ovarian masses.

Many types of skin tumors, both benign (noncancerous) and malignant (cancerous), exist. Approximately 20-40% of primary skin tumors are malignant in dogs and 50-65%

are malignant in cats. Not all forms of skin cancer in cats and dogs are caused by sun exposure, but it can happen occasionally. On dogs, the nose and pads of the feet contain sensitive skin and no fur to protect from the sun. Also, cats and dogs with thin or light-colored coats are at a higher risk of sun damage over their entire bodies.

A transmissible cancer is a cancer cell or cluster of cancer cells that can be transferred between individuals without the involvement of an infectious agent, such as an oncovirus. Transmission of cancer between humans is rare.

Contagious cancers occur in dogs, Tasmanian devils, Syrian hamsters, and some marine bivalves including soft-shell clams. These cancers have a relatively stable genome as they are transmitted.

In humans, a significant fraction of Kaposi's sarcoma occurring after transplantation may be due to tumorous outgrowth of donor cells. Although Kaposi's sarcoma is caused by a virus (Kaposi's sarcoma-associated herpesvirus), in these cases, it appears likely that transmission of virus-infected tumor cells—rather than the free virus—caused tumors in the transplant recipients.

Hemangiosarcoma is quite common in dogs, and more so in certain breeds including German Shepherd Dogs and Golden Retrievers. It also occurs in cats, but much more rarely. Dogs with hemangiosarcoma rarely show clinical signs until the tumor has become very large and has metastasized. Typically, clinical signs are due to hypovolemia after the tumor ruptures, causing extensive bleeding. Owners of the affected dogs often discover that the dog has hemangiosarcoma only after the dog collapses.

The tumor most often appears on the spleen, right heart base, or liver, although varieties also appear on or under the skin or in other locations. It is the most common tumor of the heart, and occurs in the right atrium or right auricular appendage. Here it can cause right-sided heart failure, arrhythmias, pericardial effusion, and cardiac tamponade. Hemangiosarcoma of the spleen or liver is the most common tumor to cause hemorrhage in the abdomen. Hemorrhage secondary to splenic and hepatic tumors can also cause ventricular arrythmias. Hemangiosarcoma of the skin usually appears as a small red or bluish-black lump. It can also occur under the skin. It is suspected that in the skin, hemangiosarcoma is caused by sun exposure. Occasionally, hemangiosarcoma of the skin can be a metastasis from visceral hemangiosarcoma. Other sites the tumor may occur include bone, kidneys, the bladder, muscle, the mouth, and the central nervous system.

The most frequent symptoms of transitional cell carcinoma are blood in the urine, painful urination, frequent urination and/or straining to urinate. This can look very similar to an infection of the urinary system.

Desmoplastic small-round-cell tumor is an aggressive and rare cancer that primarily occurs as masses in the abdomen. Other areas affected may include the lymph nodes, the lining of the abdomen, diaphragm, spleen, liver, chest wall, skull, spinal cord, large intestine, small intestine, bladder, brain, lungs, testicles, ovaries, and the pelvis. Reported sites of metastatic spread include the liver, lungs, lymph nodes, brain, skull, and bones.

The tumor is classified as a soft tissue sarcoma. It is considered a childhood cancer that predominantly strikes boys and young adults. The disease rarely occurs in females, but when it does the tumors can be mistaken for ovarian cancer.

In dogs, mast cell tumors are the most frequent round cell tumor.

Canine transmissible venereal tumors (CTVTs), also called transmissible venereal tumors (TVTs), canine transmissible venereal sarcoma (CTVS), sticker tumors and infectious sarcoma is a histiocytic tumor of the external genitalia of the dog and other canines, and is transmitted from animal to animal during mating. It is one of only three known transmissible cancers in mammals; other are devil facial tumor disease, a cancer which occurs in Tasmanian devils, and contagious reticulum cell sarcoma of the Syrian hamster.

The tumor cells are themselves the infectious agents, and the tumors that form are not genetically related to the host dog. Although the genome of a CTVT is derived from a canid (probably a dog, wolf or coyote), it is now essentially living as a unicellular, asexually reproducing (but sexually transmitted) pathogen. Sequence analysis of the genome suggests it diverged from canids over 6,000 years ago; possibly much earlier. The most recent estimates of its time of origin place date it to about 11,000 years ago. However, the most recent common ancestor of "extant" tumors is more recent: it probably originated 200 to 2,500 years ago.

Canine TVTs were initially described by Russian veterinarian M.A. Novinsky (1841–1914) in 1876, when he demonstrated that the tumor could be transplanted from one dog to another by infecting them with tumor cells.

Presenting complaints and clinical signs are usually related to the site of origin of the primary tumor or to the presence of metastases, spontaneous tumor rupture, coagulopathies, or cardiac arrhythmias. More than 50% of patients are presented because of acute collapse after spontaneous rupture of the primary tumor or its metastases. Some episodes of collapse are a result of ventricular arrhythmias, which are relatively common in dogs with splenic or cardiac HSA.

Most common clinical signs of visceral hemangiosarcoma include loss of appetite, arrhythmias, weight loss, weakness, lethargy, collapse, pale mucous membranes, and/or sudden death. An enlarged abdomen is often seen due to hemorrhage. Metastasis is most commonly to the liver, omentum, lungs, or brain.

A retrospective study published in 1999 by Ware, "et al.", found a 5 times greater risk of cardiac hemangiosarcoma in spayed vs. intact female dogs and a 2.4 times greater risk of hemangiosarcoma in neutered dogs as compared to intact males.

Hepatoid tumor or hepatoid [adeno]carcinoma are terms for a number of uncommon or rare neoplasms in humans, named for a visual resemblance of the cells under the microscope to those of hepatocellular carcinoma, the most common form of liver cancer. They can arise in several parts of the body, and thus form sub-types of diseases such as stomach cancer and pancreatic cancer. The WHO defines "Hepatoid carcinoma" as "An adenocarcinoma with morphologic characteristics similar to hepatocellular carcinoma , arising from an anatomic site other than the liver".

In dogs it may refer to a Perianal gland tumor, based on a similar resemblance to healthy liver cells.

Bladder cancer in cats and dogs usually is transitional cell carcinoma, which arises from the epithelial cells that line the bladder. Less often, cancer of the urinary bladder is squamous cell carcinoma, adenocarcinoma, or rhabdomyosarcoma.

Typically, either cytologic or histopathologic analysis of the suspected mass is done prior to initiating treatment. The commonly used diagnostic procedures for skin tumors are fine-needle aspiration cytology and tissue biopsy.

Cytology is an important tool that can help the veterinarian distinguish a tumor from inflammatory lesions. The biopsy technique used will largely depend on the tumor's size and location. Small masses are usually completely excised and sent to the pathology lab to confirm that the surrounding healthy tissues that were excised along with the tumor do not contain any cancer cells. If the tumor is larger, a small sample is removed for analysis and depending on the results, appropriate treatment is chosen. Depending on the tumor type and its level of aggressiveness, additional diagnostic tests can include blood tests to assess the pet’s overall health, chest X-rays to check for lung metastasis, and abdominal ultrasound to check for metastasis to other internal organs.

The exact causes for the development of canine mammary tumors are not fully understood. However, hormones of the estrous cycle seem to be involved. Female dogs who are not spayed or who are spayed later than the first heat cycle are more likely to develop mammary tumors. Dogs have an overall reported incidence of mammary tumors of 3.4 percent. Dogs spayed before their first heat have 0.5 percent of this risk, and dogs spayed after just one heat cycle have 8 percent of this risk. The tumors are often multiple. The average age of dogs with mammary tumors is ten to eleven years old. Obesity at one year of age and eating red meat have also been associated with an increased risk for these tumors, as has the feeding of high fat homemade diets.

There are several hypotheses on the molecular mechanisms involved in the development of canine mammary tumors but a specific genetic mutation has not been identified.

A mammary tumor is a neoplasm originating in the mammary gland. It is a common finding in older female dogs and cats that are not spayed, but they are found in other animals as well. The mammary glands in dogs and cats are associated with their nipples and extend from the underside of the chest to the groin on both sides of the midline. There are many differences between mammary tumors in animals and breast cancer in humans, including tumor type, malignancy, and treatment options. The prevalence in dogs is about three times that of women. In dogs, mammary tumors are the second most common tumor (after skin tumors) over all and the most common tumor in female dogs with a reported incidence of 3.4%. Multiple studies have documented that spaying female dogs when young greatly decreases their risk of developing mammary neoplasia when aged. Compared with female dogs left intact, those spayed before puberty have 0.5% of the risk, those spayed after one estrous cycle have 8.0% of the risk, and dogs spayed after two estrous cycles have 26.0% of the risk of developing mammary neoplasia later in life. Overall, unspayed female dogs have a seven times greater risk of developing mammary neoplasia than do those that are spayed. While the benefit of spaying decreases with each estrous cycle, some benefit has been demonstrated in female dogs even up to 9 years of age. There is a much lower risk (about 1 percent) in male dogs and a risk in cats about half that of dogs.

DFTD tumours are "large, solid, soft tissue masses usually with flattened, centrally ulcerated, and surfaces", which are "typically multicentric, appearing first in the oral, face, or neck regions", and are " to nodular aggregates of round to spindle-shaped cells, often within a pseudocapsule and divided into lobules by delicate fibrous septae". The disease is described as being aggressive, locally. Tumours can cover the eyes.

As Iain O'Neill notes, it also presents a "high rate of regional lymph node involvement and systemic metastasis"; of the avenues by which the cancer can spread systemically, metastasis to the lungs, spleen, and heart are observed, as is secondary "intracranial involvement". O'Neill also notes that "organ involvement and superimposed infection may... contribute to mortality." Moreover, growth of large tumours impedes feeding, and starvation is another cause of death in affected devils.

A Hürthle cell () or Askanazy cell () is a cell in the thyroid that is often associated with Hashimoto's thyroiditis as well as benign and malignant tumors (Hürthle cell adenoma and Hürthle cell carcinoma, a subtype of follicular thyroid cancer). This version is a relatively rare form of differentiated thyroid cancer, accounting for only 3-10% of all differentiated thyroid cancers. Oncocytes in the thyroid are often called Hürthle cells. Although the terms oncocyte, oxyphilic cell, and Hürthle cell are used interchangeably, Hürthle cell is used only to indicate cells of thyroid follicular origin.

Clinical signs are often vague and include weight loss, loss of appetite, fatigue, and possible jaundice.