What little we see of him background, he was in love with her future master Catrina, and the two enjoyed traveling the land and playing around. At some point Catrina found the Witchweed Potion, which changed her for the worse.
Rep is a henchman who works for Catrina but she abuses and belittles Rep, he works with the Bushwoolies as slaves and he and Catrina forces them to make the potion for Catrina herself.
After the Bushwoolies escaped, Rep was sent by Catrina to kidnap the ponies to be his new slaves, but ended up kidnapping Baby Moondancer along with Rainbow of Light to convince the Bushwoolies and the other ponies to go to work for Catrina. Rep redeems himself and ends up defeating Catrina after he manages to make Catrina renounce her evil ways and Catrina redeems herself by destroying the machine that produces the Witchweed potion, in the end Rep and Catrina end up getting married and become the loving couple they were before.
Rep is Catrina's enforcer, but just because he's trying to mitigate the damage she does and he's (initially) too dumb to take on his power-crazed boss, he longs to go back to the old days when he and Catrina were one. loving and happy couple before she was corrupted. Near the end of the special, he realizes that he is her facilitator and confronts her directly.
Powers and Abilities
He can change his shape on a whim, although he doesn't seem to be able to change his color scheme. Usually he only briefly takes other forms to deal with particular situations, such as turning into a bird to rescue a falling bushwoolie or a bull to attack Catrina.