There have been other recurring characters in the Backyardigans, those characters are:

Sherman the Worman

Sherman the worman appears in two episodes in the first season, The Heart of the Jungle and Polka Palace Party. Sherman is an intelligent yellow with orange polka-dotted worm who talks in gibberish, although Tyrone seems to understand him. Sherman is present before the backyard is transformed via imagination, and so is presumably "real" in the context of the show, although in both episodes, he is left behind in the "imaginary" world, in the company of a number of other Wormans. It may be entirely co-incidental that Tasha has not appeared in any episode that contains Sherman.

The rest of Sherman's family or race of worms, the 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 English.

Boinga, the alien baby

Boinga appears in two episodes during Season 2, Mission to Mars and Newsflash. Boinga is an alien baby that only says boinga, an important word in the alien language, according to her mother. In Mission to Mars, Boinga uses her mother's cell phone to call Earth. The Backyardigans travel to Mars to find the source of the mysterious call, which turns out to be Boinga. In Newsflash, Boinga visits Earth at the end of the episode. She comes down to the cornfield and surprises the Backyardigans. 

Special non-recurring characters 

In the two-part International Super Spy, Cyndi Lauper provides the singing voice for the song "The Lady in Pink". In Mission to Mars, Alicia Keys provides the voice for the alien mom. Boinga, the alien baby, is voiced by Shakira Lipscomb. In the UK version of the show, the baby alien is voiced by Maria Darling, but this has the unfortunate effect of having the baby sound exactly like Tasha (also voiced by Darling). Alicia Keys vocal is not affected by the normal "Anglicization" that is common in the UK version of the show.

t's Great to Be a Ghost and Secret Mission include voices near the end of the episode that do not come from any of the main characters. They're rumored to have come from Evan Lurie, one of the show's composers. There is also an announcer voice at the beginning of the newscast in Newsflash.