From Muppet Wiki

Arnold is a monster who appeared in a series of Munchos potato crisp commercials alongside Fred, the Munchos spokesman.

A slightly smaller antecedent of Cookie Monster, the ravenous Arnold spoke in a simplistic dialect, frequently crying out for "MUNCHOS!" He provided a contrast to the more staid spokesman, even going so far as to break through walls in his search for the tasty snacks.