He first appeared in the first season episode "Close Encounters of the Frog Kind." Baby Kermit discovers he's an uncle because his "big sister" had babies. Robin wonders if he'll look like the "big frog" Kermit when he grows up.
Robin appeared again in the fourth season episode "Adventures in Muppet-Sitting."
Robin, like Baby Animal, only speaks in baby babbles, but for one brief scene in "Close Encounters of the Frog Kind," his mouth movements sync up to one of his thoughts.