Sherman
Two episodes in the first season, The Heart of the Jungle and Polka Palace Party, feature a character called Sherman the Worman, an intelligent yellow-and-orange-polka-dotted worm who talks in gibberish, although the characters seem to understand him.
Wormans also appear in the third season episode What's Bugging You?, though these Wormans are somewhat different, in that their typical 'speech' is meeping instead of gibberish and they are capable of speaking (high-pitched) English. A worman is also featured in the Fourth Season episode Attack of the Fifty-Foot Worman, though this one appears incapable of speech.
Read more about this topic: The Backyardigans, Characters and Voice Cast, Occasional Characters