Both. "Don't" is heard in "Stimpy's Big Day/The Big Shot," and "didn't" is heard in "Fire Dogs," and "Marooned." In Season II we have a new variation in "Out West:" "WELL sir, I don't like it." Another variation is in "Rubber Nipple Salesmen:" "WEll sir, I don't think I have a use for rubber nipples!"
By the way,
Mr. Horse has appeared as a cameo on several cartoons but has starred in two bumpers. Yes sir, those appearances were Emmy material! :)
FYI: He is on the picture in Anthony's bedroom in "Visit To Anthony" and on Ren's wall (behind Stimpy and the flensing machine) in "Nurse Stimpy."