Just some AT theories.
Finn was raised by Hyoomans
This one is pretty clear. Finn wears a hat like a Hyooman, so it's quite likely he was raised by them.
Marcy was turned into a vampire by the vampire king
She was born a demon to a demon, yet she still has fang marks. This must mean she got them from the Vampire King she defeated.
Jake the Maniac
Card Wars and The Gut Grinder have proven that Jake is too unintellegent to tell the difference between fantasy and reality. The mere suggestion that he was the Gut Grinder turned him into it, when clearly any sane, logical person would have known that the gut grinder was a separate being. Also, Jake went nuts over a card game. I mean, I've gotten annoyed over card games too, but never at the level Jake got to.
Gunter is behind the Great Mushroom War
Gunter is clearly an evil genius who is behind all the events of Adventure Time, so it's most likely that him or a parent of his started the GMW