The onager is a wild ass. On March 25 the onager brays twelve times to signal the spring equinox; he brays both in the night and the day, and the number of brays marks the hour. The male onager is the leader of the herd, and to keep all the females for himself, he castrates any male young that are born. Because of this, the female onager will try to hide her offspring from the male.

The onager represents the devil, who on seeing sinners converted brays in anger at his loss. Philippe de Thaon says that day represents good people, night bad people, the hours the number of people, and the equinox eternal heaven and hell.