Throughout the Ghostbusters movies, the ghosts we see all appear to be ghosts of humans - with the exception of Slimer, which is a green blob with no legs and spindly arms.
What sort of creature (or human) was Slimer before he became a ghost?
Dan Ackroyd described him in the "making of" book as being literally the embodiment of the sloth and gluttony that you find within the walls of a swanky hotel. It stands to reason that he didn't exist (e.g. he's not the ghost of anything) until he came into being:
"So, in reading the literature and reading about full head and torso apparitions, I found out that it is very rare that you see a full figure -- it is usually just a hint of the former being. The Onionhead is a vapor -- a kind of confluence of stored up psychic energy. He's an accumulation of spirits that haunt this hotel, and he just doesn't want to leave."
Both Ackroyd and Joe Medjuck have stated in various interviews that Slimer is the ghost of their long-time friend John Belushi. Again, to quote from the "making of" book;
One day, during preproduction, we were all sitting around talking about the Onionhead concept, and Ivan remarked that the character was sort of like Bluto in Animal House -- like the ghost of John Belushi, in a way, Danny, who was obviously a good friend of John's, never argued with that. Even so, we never officially said that and we never mentioned it in the script. It was just one way to look at the character, because Onionhead's grossness is like Bluto's in Animal House. We certainly never expected anyone to recognize him as such, although somehow the word did get out and we received some calls from a few newspapers saying they'd heard we had the ghost of John Belushi in our movie.