He was voiced by James Patrick Stuart.
In "The Last Donnycorn", Petercorn, along with the other members of The Four Unicorns of The Apocalypse ran around Unicorn World, causing evil. They were trapped inside of a dark magic dark mirror of darkness, as punishment, and locked away there for years to come, before Donny came along and accidentally set them all free.
The Four Unicorns of The Apocalypse struck Unicorn World and the leader of the team, Stanicorn constructed an evil empire, where he would steal all the magic from the other unicorns and take over Unicorn World. Meanwhile, Petercorn and the other unicorns stood guard at the front of his throne.
Petercorn fought with Blossom, by taking out his electric guitar and blasting her with insanely loud music. Blossom used her ice breath on him, freezing the guitar, so that it was no longer playable. The ice shattered, and the guitar broke with it, giving Blossom the ability to go right ahead and kick his butt, the old fashioned way, with her fists and feet. Petercorn was not good with non-music related fighting, so this was an easy defeat.
After the death of Stanicorn, The Four Unicorns of the Apocalypse disbanded and Petercorn, as well as Joeycorn and Tanyacorn became nice guys. They changed their leader to Gretel, who led them in the right direction.