I assume the idea is that Jorb doesn't want people to kite Trolls with horses. Jorb's solution: Trolls go berserk when they see a horse.
However, this creates massive trap for people who don't specifically know not to ride a horse when mining - or who know, but make the reasonable assumption that the troll will just kill the horse and let them escape.
Possible alternative solutions:
A) Give trolls an AoE attack that causes nearby horses to instantly KO.
B) Give trolls an AoE attack that removes all pony power from all nearby horses.
C) Give trolls a massive speed boost when a horse is nearby.