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".


* An iugerum is an area 240 feet in length and 120 in breadth.

** '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)