The term arose from the formerly widespread belief, present in many European folk traditions, that such malformed creatures were the product of the sinister influence of the moon on fetal development.
The term is also used as a name for a character in the computer game Discworld Noir, head of the cult of Anu Anu; and as a nickname for Seoman Snowlock from Tad William's Memory, Sorrow, and Thorn books.
In Gerald Durrell's fantasy book The Talking Parcel, the mooncalf is a large fictional creature with the head of a cow and body of a snail which lives in the land of Mythologia. The mooncalf has three spigots on the back of its shell for warm milk, cold milk and cream and leaves a trail of mooncalf jelly which can be willed (by the mind) into different shapes.
In Bruce Cordell's Dungeons & Dragons's adventure Heart of Nightfang Spire, the mooncalf is a large fictional creature possessed of bat wings and many tentacles that are sometimes encountered on "mountain tops, the tips of tall hills, and generally lonely, desolate places."