According to Camp Underpaw's Guide to Mythical Creatures: A giant, vicious creature shaped like a bull with a mane of a horse, horns that curve backward, and fiery dung. Extremely easy to track by its excretions, but approach with caution. Bring nose plugs and a fire extinguisher.
Camp Underpaw's Bonnacon Edit
Camp Underpaw, the Tracker camp Matthew Kahn previously went to, owns a bonnacon. Two other participants filmed Matthew riding it, and its excretions (or dung) ended up setting fire to Kiri's enclosure, allowing her to escape. It is unknown if the camp kept the bonnacon, but it is possible they released it so it would not free any other creatures.