I think he's a character Peter Straub created in one of his books.
But in the Dark Tower, although he never makes a physical appearance, he leaves graffiti all over the place, in unlikely areas like in the tunnels beneath the Dixie Pigg and in various alternate versions of NYC.....so Bango Skank gets around.
I personally love it when i see Bango Skank written places around where I live. It makes me smile knowing there are other Dark Tower fans around.