Arnie the Alligator made his debut in The Muppet Movie when he scared away Bernie the Agent, who was just helping Kermit to rethink his destiny. In this appearance, Arnie was a realistic-looking animatronic alligator puppet.
In Kermit's Swamp Years, Arnie appeared in Muppet form, as performed by John Kennedy. He served as a kind of bodyguard and elder mentor to Kermit and his young friends, Croaker and Goggles. The puppet used for Arnie here was one of a pair of alligator puppets originally built for Muppet Treasure Island.
Kermit's Swamp Years gave a brief explanation as to why Arnie tried to attack Bernie in The Muppet Movie: Arnie has several misconceptions about humans and views them with suspicion.