I think the media labeled him that. In his pre-Ali days I also remember some reporter calling him 'Gaseous Cassius'. Presumably, again, for his talkative nature rather than any bowel impediment (Oh, man, I knew I shouldn't have eaten all that cabbage)
It should be noted, however, that The Rabbit, The Acorn and as @williams0504 mentioned, 'Vito the Mosquito' weren't actually nicknames, but name-calling.
Think it was but after Ali nicknamed that he embraced it probably the only fighter to do so Ali's names weren't the nicest some included the ugly bear, the gorrilla, the rabbit, the mummy funniest probably Dracula referring to Spinks gap tooth
The one that has stuck in the conscience is ‘The Bear,’ for Liston. People think that was really his nickname, when in fact, he was if anything known in his time as, ‘The Night Train.’
The West Coast is a kinder, gentler place. Little Red Lopez Gato Gonzalez Buddy Baer (who would be scared of fighting their buddy?) Couple of Golden Boys Etc