Buck

reflex

Most monsters that serve as mounts can attempt to buck off unwanted or annoying riders, but most mounts will not use this reaction against a trusted creature unless the mounts are spooked or mistreated.

Trigger A creature Mounts or uses the Command an Animal action while riding the monster.

Effect The triggering creature must succeed at a Reflex saving throw against the listed DC or fall off the creature and land Prone. If the save is a critical failure, the triggering creature also takes 1d6 bludgeoning damage in addition to the normal damage for the fall.