Slimer is a green-colored specter who made his debut in Pooh's Adventures of Ghostbusters.
Pooh's Adventures of Ghostbusters
While little is unknown about who he was in life, Slimer was known to be a servant of Gozer in the afterlife. At one point, he manifested at the Sedgewick Hotel in New York City and caused disturbances there. His antics eventually drew the attention of the Hundred Acre Crew and the Ghostbusters. He was subsequently released, however, when Walter Peck shut down the containment system and released him along with all the other ghosts they'd caught.
Following the Ghostbusters' triumph over Gozer, Slimer reappeared to the Hundred Acre Crew. They attempted to catch him in a trap, but the Good Fairy encouraged them to spare him. Just to annoy Rabbit, Tigger gives him his name.
Pooh's Adventures of Ghostbusters II
Slimer stuck around with the Ghostbusters even after they'd gone out of business. When they'd returned to action, the onionhead hung around the firehouse, even sometimes eating Louis' lunch. And when trouble returned to the city in the form of Vigo the Carpathian, Slimer acted to help save the day by bringing Louis to the scene of the Museum, where he helped break down the barrier surrounding the Museum while the others inside fought and beat Vigo. It was after this that Slimer decided to join the Hundred Acre Crew full time.
Pooh's Adventures of Scooby-Doo and the Witch's Ghost reveals that, although he was a former servant to Gozer and the Terror Dogs, he wasn't able to detect any evil from Ben Ravencroft. Mostly due to lures of food, as usual.