When Peter Puppy transforms, gone is the small cuddly exterior of his usual form. Through a painful and pretty unpleasant looking transformation, Peter mutates into a massive hulking monster of a dog. Unable to control his anger and frustration he takes it out on anyone that happens to be near!
Peter has tried to control his emotions, in efforts not to morph into his demonic alter-ego. However has never been successful. The reason he mutates in such a way is unknown.
Not a dog from Earth...The dark barren planet that he originated from is battered daily by asteroid showers, raided by passing aliens, and overrun with strange and dangerous octopus-like organisms among other things.
On a planet where there are life forms that survive by rooting themselves deep underground, and capturing small defenceless creatures on the surface - then squeezing them to death...perhaps it is not surprising that Peter does mutate in such a way. As his own personal bodyguard, he has little difficulty in protecting himself.