Buffalos

Buffalos are most common large herbivorous herd animal.
They definitely descend from the old-days american buffalo (bison), although mutated, and it is believed there was some kind of mixing with large breeds of domestic cows.
They weight between 500 and 1300 kg, and are pretty much the same as an american buffalo, with one exception: They have a set of 4 fearsome horns, a pair of large horns above, resembling the ones displayed by ancient asian water buffalos, and a pair of medium sized horns below, similar to those of scottish highland bulls.
Bulls are quite aggressive, and compete among them for dominance. Females on the other hand have smaller horns, and are rather peaceful, but protective of their calves.
They live in large herds, preferring open spaces such as grasslands and prairies.
When confronted against a threat, they react differently. If they are confronted against a single powerful threat, they will form a line of many strong bulls and stomp the ground, and threaten back, and after a few moments, they will charge against the threat, followed by a stampeding herd.
If they are confronted against a distributed threat, a threat from the skies, are caught by surprise, or against an unmanageable threat (fire), they will panic and stampede away from the threat.