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 to Kermit and his young friends, Croaker and Goggles. The puppet used for Arnie here was one of a couple 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: it is explained that Arnie has several misconceptions about humans.