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.
Although Jim Henson voiced Arnold in one ad, Frank Oz performed the monster as he would perform Cookie Monster later on.[1]