Bubbles was a pet goldfish owned by Elmo. In Episode 3587 of Sesame Street, Bubbles' death is the source of Elmo's unusually somber mood. Telly and Miles allow Elmo to cry it out before getting him a new goldfish (which Elmo names Miles-Telly) to cheer him up.
Elmo briefly references Bubbles in Episode 3569, saying that his goldfish is one of the things he loves.