Peter Puppy is a supporting character and close friend of Earthworm Jim.
Peter Puppy turns into a large monstrous dog whenever he is hurt or scared. This trait is very similar to that of Bruce Banner as the Hulk. While he is in this form, he will attack Earthworm Jim or anything that resembles him (as seen when he once attacked Psy-Crow, who was disguised as Jim at the time). After a short while he will change back to his natural form, unaided. The animated series expanded upon this, adding that tickling Peter would cause him to belch, then change back to normal.
In the Earthworm Jim video games, Peter has the ability to change into a large monster when threatened. He is from another planet on which Jim crash-landed after being sucked through a warp hole. Jim's Pocket Rocket was badly damaged in the crash, and Peter helped Jim escape the planet in exchange for being helped to safely return to his house. In Earthworm Jim 2, Jim chases Psy-Crow to Peter's planet following Psy-Crow's kidnapping of Princess What's-Her-Name. Psy-Crow has taken Peter's six hundred relatives hostage and is throwing them from the window of a tall building. Jim must save the puppies by bouncing them to safety via a large marshmallow. However, should you drop too many puppies at once, Peter will "unleash" his wrath and severely damage you.
Peter's planet is battered daily by asteroid showers, raided by passing space dogs, and overrun with strange and dangerous octopus-like man-eating plants. His violent transformations may be seen as a defense mechanism to these constantly present dangers.
In Earthworm Jim 3D, Peter plays the role of Jim's Super Ego. This Peter is clearly based on the cartoon version.
The animated series changes Peter's origin considerably, portraying him as an ordinary dog. His appearance changed dramatically as well, with the artist giving Peter an unbuttoned teal shirt and black shorts as opposed to walking around nude. His fur was changed from purple/pink to white to make him look more like a real dog. A coward by nature, he was picked on by the other neighborhood dogs. One day Peter decided to prove his worth by performing the Taboo of the Dogs, attacking a running vacuum cleaner, something few dogs have attempted. His fear turns out to be well-justified as vacuums are canine-specific trans-dimensional portals. Peter is flung through time and space and eventually lands on the Planet Heck where Evil the Cat places a curse on him. The curse gives Peter human intelligence and his anthropomorphic form (and his nifty shirt), but also causes him to transform whenever he is hurt or scared. He eventually finds Jim and the two become good friends, with Peter serving as Jim's unofficial sidekick. As such, he had a much larger role in the cartoon series than in the games, appearing in almost every episode.
Peter Puppy is quite timid by nature and has a hard time controlling his transformations. He can often be heard chanting to himself, "I must not fear, fear is the mind killer, fear is the little death that brings total oblivion" (a modified litany against fear, From Frank Herbert's Dune series). Despite his cowardice, Peter is always willing to accompany Jim on his adventures, acting as Jim's consultant when Jim goes astray, as well as providing Jim with moral support. Whilst Peter does have a kind of hero worship for Jim, he does often point out certain inconsistencies of Jim's, like when Jim called Peter a kettledrum, which Peter takes as an insult on his weight; or Jim's ability to use aromatherapy despite having no nose. This invariably makes Jim say, "Don't question me" in a threatening tone of voice.
Peter is very artistic and has dabbled in many different forms of expression such as interpretive dance, gourmet cooking, and play acting. He is also fond of professional wrestling, licking stamps for the taste of their glue, and playing in garbage. Peter enjoyed eating haggis until he discovered its ingredients and method of preparation. As a result, his greatest fear is a Scottish chef and he now cannot eat haggis without getting sick, though he has been tricked into eating it on numerous occasions. This is a running gag in the animated series.
- Peter's voice actor in the animated series was Jeff Glen Bennett, who also voiced the narrator.
- Peter's fur is light pink (light yellow in the cartoon) when in normal form, but when he gets hurt or scared or angry, his fur turns dark pink in his monster form.