Let's touch on some topics really quickly here:
Ghosts are not simple manifestations of something that wasn't in existence before. In 32 years of paranormal research, I've never once read such a theory as that, and there isn't anything that backs that up at all, either.
Ghosts/apparitions are the disembodied spirits of once living creatures. Check any of your textbooks on parapsychology and that's basically your definition. There has never been any documented case where a ghost appeared that was of something that was never alive to begin with.
Except in Hollywood.
Anyways, go to
http://www.aspr.com/ , or go directly to the right textbook here:
http://www.aspr.com/general.htm#irwinIf you have reference to support that ghosts don't have to be of once living people that denies the years over years of official studies by parapsychologists, please link. I'm willing to read that information as well.
Next, the “Joke” was that they DID base The Onionhead Ghost on John Belushi, who, as they say, lived large and basically was the living definition of “over-kill” by the mass consumption of food, drink, and drugs. You all know that John Belushi was gluttonous, and his friends today still say he was. It was just who he was.
So, who is this in the book?
We all know and love Slimer, but that's the name given to him in the Real ghostbustes cartoon, and later in the RGB comic books. Then, in GBII he was named Slimer. But, this comic series takes place after the first film and doesn't consider the existence of GBII or any of the animated series, and so I call this green ghost “The Onionhead.” And I love it. I love the way they did the art on him in this book.
But, you also ask how he ends up in the book? Remember at the end of the movie just before we fade to dark and credits he's free and flying through the streets of New York? That's how he's ended up in the book.
I don't have the original novel written about the first film, so I'm not sure what insight they give in that book about The Onionhead as he was known back then. But, according to the book Making Ghostbusters, and my book is loaned out so I can't be 100% for sure, but I think the John belushi reference is made there, also.