Series creator John Kricfalusi created the characters during his stay at Sheridan College and they first appeared on film in the pilot episode "Big House Blues".

Ren is a scrawny, emotionally unstable, and psychotic "Asthma Hound" Chihuahua, and his best friend Stimpy is a dim-witted, good-natured Manx cat.

Martin "Dr. Toon" Goodman of Animation World Magazine described Ren as scrawny, dyspeptic, and violently psychotic, who loses his mind occasionally in a cumulative process resulting in his becoming, in Goodman's words, a "screaming klaxon, neon-pink eyes dilating into twin novae inches above his jagged, monolithic teeth.

"[1] Andy Meisler of The New York Times described Ren as "adventurous", "intelligent", and "emotionally brittle".

A running gag involves Stimpy being confused with and treated like a dog, due to his unusual appearance not resembling a cat.

Kricfalusi originally voiced Ren in a manner that he describes as "a bad imitation of Peter Lorre".

Cat is a 3-year-old anthropomorphic mildly overweight red and white, manx cat, with a blue nose, purple eyelids, no tail, paws with gloves and four fingers that have fingernails, human-style buttocks, flat human feet, four wiggly toes and a brain the size of a peanut.

[2] Stimpy's trademark facial expression is a blissfully ignorant smile with his tongue hanging out.

[7] Stimpy was inspired by Kricfalusi's former girlfriend and the series' co-developer, Canadian animator Lynne Naylor.

Andy Meisler of The New York Times says he feels a fixation for "sensory pleasures of fresh kitty litter".

West will reprise his role as Stimpy (along with voicing Ren) in Comedy Central's reboot of the show.

[6] Kricfalusi originally created Ren and Stimpy as the pets of George Liquor and Jimmy the Idiot Boy.

[11] Kricfalusi received inspiration for Ren from a black and white photograph of a chihuahua in a sweater next to a woman's feet.

[13] He received inspiration for Stimpy's design by a Tweety Bird cartoon called A Gruesome Twosome where the cats in the animation had big noses.

Dennis adds that in other situations Ren and Stimpy are "read more appropriately" as coworkers, enemies, friends, and house pets.