Pliny speaks of a creature, which he calls the bonasus, of Paeonia (in Asia), having the mane of a horse, but in all other respects the look of a bull, with horns so curled as to represent no danger, but capable of emitting excrement whilst running away, of such noxiousness that pursuers were scorched as if by fire.
Subsequent accounts have added a horse's tail to the mane, and poisonous gas to the excrement, and the variant name of bonacon. Another beastiary spelling is bonnacon.
The bonacon appears in English heraldry and only from the mid-16th century. Its sole apparent appearance is as a crest granted to Richard Chandelor in 1560, with the head the only depicted part. There seems to be no other heraldic use outside of that instance.
Bonacons in the SCA
Wilhelm von Schlussel ruled that the bonacon was too offensive to be used as an heraldic charge in 1980. More recent speculation about the charge implies that a single use as a partial monster in a crest does not set a pattern of heraldic use appropriate for its inclusion in SCA heraldry, although no specific ruling has been made on this.