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.
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.
It is described in Fantastic Beasts and where to find them as a shy, nocturnal creature with a smooth, pale-gray body, bulging eyes, and four spindly legs with large flat feet.
[3] In the film of the same name and its sequel, Newt Scamander's collection of creatures in his suitcase includes a herd of mooncalves.
In the fourth episode of the seventh season of The Big Bang Theory, Sheldon calls Amy a "dewy-eyed mooncalf".