Slimer is a green translucent blob, with no feet, two arms and several chins. Slimer's personality is one of tremendous gluttony, referred to as a "disgusting blob" whose original moniker was simply The Onionhead Ghost. In the first movie, Slimer was voiced by the films director Ivan Reitman.
Dan Aykroyd has said that he considered "Onionhead" to be, partially, the ghost of late actor/comedian/friend John Belushi.
The name "Slimer" was not bestowed upon the creature until the creation of the children's cartoon show The Real Ghostbusters. He was such a hit that the show was later changed to Slimer and the Real Ghostbusters and geared towards an even-younger audience.