According to Japanese beliefs, Mumashika are comical-looking yōkai with a head resembling a one-eyed horse. A single horn sprouts from the back of their skull, and fangs protrude from the side of their mouth. Their bodies resemble those of deer. Their clothing is loose and billowing, while they wave their hooves in the air with a playful silliness.

Mumashika first appears in the Hyakki yagyō emaki, and has been copied into numerous other books and scrolls since its debut. Although its name is presented as mumashika, it is written with kanji that are most commonly used for "baka" — an insult which means foolish or stupid. Like the other yōkai in this scroll, it is presented without commentary, so the artist’s original intent for it can only be guessed. Perhaps it is a spirit which possesses people and causes them to behave foolishly, or perhaps it is merely a cartoonish representation of the concept of foolishness. Or perhaps the artist is just making a pun based on the fact that the word for foolish is written with the characters for horse and deer.