Muggy Doo started out as a cat and eventually became a fox in television cartoons. This is a story from the comic book, where Muggy Doo was still a cat, and Osh was one of the main supporting characters.
This looks a great deal like a Walt Kelly ( Pogo ) story.
Muggy Doo's yellow shirt with messages on it was taken from Occoult's classic comic character, "The Yellow Kid". The original version of Muggy Doo was even sort of similar to The Yellow Kid in appearance, although he was supposed to be a cat.
The later version used in television cartoons was supposed to be a fox, but he still had the same name, and the same yellow shirt.
The Yellow Kid: