Miles, Clement A. (1912). Christmas in Ritual and Tradition. from Chapter XIII: Masking, the Mummers’ Play, the Feast of Fools, and the Boy Bishop, "Mr. Chambers's theory is that the ass was a descendant of the cervulus or hobby-buck who figures so largely in ecclesiastical condemnations of Kalends customs."
This article incorporates text from a publication now in the public domain: T. J. Crowley (1913). "Feast of Asses". In Herbermann, Charles (ed.). Catholic Encyclopedia. New York: Robert Appleton Company.