"Mumbles" in the syndicated cartoon strip Dick Tracy, as well as the movie based on it, where he was played by an uncredited Dustin Hoffman. In the movie, his speech appears to be modeled on that of Marlon Brando. In the post-Chester Gould period of the strip, writer Max Alan Collins created Merky, a man whose speech is totally incomprehensible, being made up of original symbols.
Snuffles, the cat from Pearls Before Swine, only talks in meows. Somehow, everyone else can understand him.
Woodstock and the other birds from Peanuts; their dialogue is represented with hash marks. Snoopy, however, understands them.
The Imp in Little Nemo in Slumberland. In the words of the Princess: "He can't understand English or any other language!" (He can talk to lions, however). In his own words: "Gimmel iggle ip soggle opp sog!"
In a Garfield Sunday strip, Odie stands on a fence and tells a joke to an invisble back alley audience. However, since Odie can't talk (not even "thought speech", like Garfield), he barks the joke. Not only does the audience understand him, but they think the joke is really funny.
During his first appearances in Get Fuzzy, Mac Manc McManx spoke almost completely unintelligibly. Not by virtue of having muffled speech or talking like a real animal, mind you, but simply because of the unholy way he mixed and matched however many Brittish slang dialects one could think of into each of his sentences. This was later toned down to make what he said somewhat more comprehensible.
Mac (from his first appearance): Been havin' a butcher's for your flat all day, mate, thought I'd cocked summat up! I'm well knackered, I can tell you! I could do with a bevvy and a kip! [...] Plus. Some bloke diddled me brolly in the queue for the khazi back in Blightey.