I know of:
Brool (?) the Minstrel appeared in a few episodes. In "The Argument" he was a customer who was always arguing and needed an argument. In "Goodbye", he sang in the background.
Beastie (Tosh's pet) appeared in "Goodbye."
The Inkspots appeared in "Dun Dun Dun", "Goodbye", "The Mushroom's Always Greener," and "Silly Steven".
The creature with a trumpet for a nose appeared in "Goodbye", he was a magician in some episode, and he appeared together with the pink furry trumpet creature from TMS in another episode in which they wanted to trade colours.