Appearing about 240 BC, incendiary pigs were an interesting weapon thought to have been utilized in ancient Roman warfare. The concept was to cover the pig in tar and a flammable substance and, when lured close enough to the advancing or defending enemy the pigs would then be lit on fire. The hope was that the pigs would run uncontrollably into the ranks of the opposing force, causing a certain level of confusion. In any event, the idea was to make use of the weapon as a psychological tool to harass or scare the enemy into submission, or break his concentration at the very least.

More here. You can buy models of them (unlit) from EM4 miniatures.