The Mooncalf is a shy creature that only comes out of its burrow during a full moon. The Mooncalf has smooth, pale grey skin, and four spindly legs that end in large flat feet. The Mooncalf also has bulging eyes that sit on the top of its head.
Once the Mooncalf is in the moonlight, it performs complicated dance moves standing on its hind legs. It is believed that this is part of the Mooncalf mating ritual, but also has the side effect of creating geometric patterns in wheat fields that confuse Muggles.
Mooncalf dung, if harvested before the sun rises, will make magical plants grow fast and strong.
In modern terminology, a "mooncalf" is a foolish person. The term originally meant a deformed calf born under the malign influence of the full moon. In the 17th century, it came to mean a deformed baby, and later, a hideously deformed individual; as in Shakespeare's Tempest, when Stephano calls Caliban a "mooncalf."
The "geometric patterns" mooncalves make could be a reference to crop circles, believed by some to be made by UFOs.