bonacon

In heraldry, a bonacon or bonasus is a bull-like creature with a horse's mane and serrated horns curving in on each other. Its mutant power is the ability to eject its excrement at its enemies thus causing them to be burnt up as if by fire.

It is found in ancient bestiaries.

A Bonacon is an heraldic monster with the appearance of a bull. However, it has small horns curled in like a ram's, a horse's tail and a mane. The head of a Bonacon appears in English heraldry at least twice. In 1560, it was granted to Richard Candelor of Walsingham in Norfolk, and a Bonacon's head was also granted to Hugh Hollynside of Cheshire in 1561. This beast was also known to Aristotle and Pliny, and features in the Mappa Mundi in Hereford Cathedral.

It is generally assumed that the Bonacon is not commonly seen in heraldry, as the creature's most unique feature is the manner in which it defends itself. Upon being pursued, it raises it's tail and emits such a noxious gas that trees within three acres catch fire. The enthusiasm of any pursuer would be dampened by this powerful emission. The bonacon is normally depicted standing on all fours with it's tail raised and the emission radiating.

This node would not be complete without Pliny the Elder's original description, as given in his Natural History:

Tradunt in Paeonia feram quae bonasus vocetur, equina iuba, cetera tauro similem, cornibus ita in se flexis, ut non sint utilia pugnae. Quapropter fuga sibi auxiliari reddentem in ea fimum, interdum et trium iugerum longitudine, cuius contactus sequentes ut ignis aliquis amburat.
(Pliny's Natural History: Book 8, xvi - 40)

Or, in English:

In Paeonia (somewhere in Asia) there is found a beast, the bonasus, that has the mane of a horse, and otherwise resembles a bull. It has horns that curve back so they cannot be used for fighting. When attacked, it runs away, while releasing a trail of dung that can cover 86,400 square feet*, contact with the dung burns pursuers as though they had touched fire.

Webster1913 now defines a bonasus as an auroch or European bison; the beast of heraldry has taken on a different spelling, bonacon or bonnacon**. The horse tail and the poisonous gas accompanying the poo are also later embellishments. Although it has been used on heraldic crests in the past (first recorded appearance was in 1560, granted to Richard Chandelor -- perhaps a case of ancient sarcasm?), the Society for Creative Anachronism no longer allows them, as it was deemed "too offensive".

** 'Vilde kow' is also sometimes given as a synonym, but as far as I can find this is simply an early form of 'wild cow'; probably not limited to the bonasus.

References:
http://penelope.uchicago.edu/Thayer/L/Roman/Texts/Pliny_the_Elder/8*.html
http://bestlatin.net/zoo/bonnacon.htm (pictures!)
http://www2.kumc.edu/itc/staff/rknight/monsters.htm
http://www.eaudrey.com/myth/bonnacon.htm
http://cunnan.sca.org.au/wiki/Bonacon
http://en.wikipedia.org/wiki/Iugerum
http://talossa.proboards32.com/index.cgi?board=collegeofarms&action=display&thread=1155846110 (:-D)