- “Hmmmmmmm.......No sir, I don't like it.”
- ―Mr. Horse
Mr. Horse is the most frequently recurring character in the series. He oftentimes acts as a 'bit character' carrying on many different roles, depending on the plot or the episode. He is voiced by series creator John K. who also voices Ren and several other characters. After he was fired, Billy West took over the role. When the show was briefly revived as an Adult Party Cartoon, John K. returned to voice him.
Mr. Horse is, as his name implies, a horse with a gray coating and a dark blue mane. His appearance remains overall the same, but depending on the episode or the part he is portraying, he occasionally wears clothes. In all of his appearances, he is seen with horseshoes.
He first appeared in "The Big Shot," testing out the difference between litter box A and litter box B - the Gritty Kitty formula. He first tests litter box A and replies "No sir, I don't like it" (which became his catch phrase).
He also appeared in "Rubber Nipple Salesmen" when Ren and Stimpy went to his door to sell their rubber nipples, but he was dressed in them. He believed them to be the FBI, but Ren assured them that they weren't. It is implied that he might've kidnapped a walrus, as he says that the pair can try and sell the rubber nipples to him, who whispers, "call the police."
He was a preliminary judge in "Dog Show" as if he didn't like the animal, he'd cut them out of the competition by forcing them to be eaten by a monstrous dog.
"In The Great Outdoors", his voice can be heard after Ren is transformed into a horse by food pills, saying his classic line, which turns out to be the final time that Mr. Horse would be voiced by John K, as John K. was fired soon after. Billy West would soon take over his role, along with majority of John K.'s other roles.
He and Ren appear in various episode shorts in Season 2, entitled "World Crisis With Mr. Horse" where he and Ren portray news reporters.
He also made many cameos afterwards, such as at Ren's party in "Jerry the Bellybutton Elf".
In "My Shiny Friend" he appears on television in The Muddy Mudskipper Show, telling Muddy not to hit him with a mallet, which he would end up doing, regardless.
In "Insomniac Ren" he appears as one of Ren's golfing buddies. It appears in the post-Spumco era it appears that he is close friends with Muddy, as they make a variety of appearances together.
- He is the most recurring character in the series.
- He makes two appearances in the Adult Party Cartoon, with his first appearance in "Ren Seeks Help" as a therapist that Ren goes to and in "Stimpy's Pregnant" in which he appears as a doctor known as 'Dr. Precision.'