A mooncalf (or moon-calf) is a monstrous birth, the abortive fetus of a cow or other farm animal. The term was occasionally applied to an abortive human fetus.

The term derives from the once widespread superstition, present in many European folk traditions, that such malformed creatures were the product of the sinister influence of the Moon on fetal development.

A monstrous birth, variously defined in history, is a birth in which a defect of some sort renders the animal or human child monstrous. Such births were often taken as omens, signs of God, or moral warnings, but besides these supernatural or religious explanations, medical explanations were also given, in which often the mother's state of mind or her sexual behavior was responsible for the deformed fetus. In early and medieval Christianity, monstrous births posed difficult theological problems about humanity and salvation; in the early modern period the interest shifted toward scientific inquiry.

An early reference to monstrous birth is found in the apocalyptic biblical text 2 Esdras, where it is linked to menstruation: "women in their uncleanness will bear monsters." Monstrous births are often placed in a religious context and interpreted as signs and symbols, as is evidenced in the 1493 "Nuremberg Chronicle". According to David Hume's "The Natural History of Religion", they are among the first signs that arouse the barbarian's interest. Monstrous human births raise the question of the difference between humans and animals, and anthropologists have described different interpretations of and behaviors toward such births. Among the East African Nuer people, monstrous births are acted on in a way that restores the division between the categories of human and animal: "the Nuer treat monstrous births as baby hippopotamuses, accidentally born to humans, and, with this labelling, the appropriate action is clear. They gently lay them in the river where they belong."

Whether monstrous births were natural, unnatural, or supernatural remained a topic of discussion. Saint Augustine held that nothing "done by the will of God could be contrary to nature," whereas Thomas Aquinas considered some miracles to be against nature.

The term came to also refer to any monstrous or grotesque thing. Shakespeare, for instance, used the term to describe Caliban, the deformed servant of Prospero, in "The Tempest".

In H. G. Wells' 1901 novel "The First Men in the Moon", large creatures domesticated by the Selenites are referred to as "mooncalves."

Mooncalf is used as a derogatory term indicating someone is a dullard, fool or otherwise not particularly bright or sharp. For example, W. C. Fields in "The Bank Dick" (1940) advises his prospective son-in-law to avoid being a "mooncalf" by buying shares he has been conned into believing are worth much more than the proffered price.

Mooncalf is also the name of a species of magical creatures in the world of the Harry Potter series. It is described in "Fantastic Beasts and Where to Find Them" as a shy, nocturnal creature with a smooth, pale grey body, bulging eyes and four spindly legs with large flat feet. Mooncalves perform dances in the moonlight, and are apparently responsible for crop circles. In the film of the same name, Newt Scamander's collection of creatures in his suitcase includes a herd of Mooncalves.

Wilfred Maxwell, narrator and protagonist of occultist Dion Fortune’s 1938 novel, "The Sea Priestess", refers several times to a mentally handicapped character who falls into the sea and disappears as a "mooncalf".

The Commodore refers to Enoch Thompson as a mooncalf in conversation with sheriff Lindsay in season 5 of "Boardwalk Empire".