According to a few websites I dug up, Dispater was a Roman death god. *Shrug*
Belial is a Judaic demon.
As for Mephistopheles, he was in the old demonology (whatever that is) one of the seven chief devils, in power second to only Satan. Apparently, the myth of Faust dates back a long time before Goethe or Marlowe...
Baal, aka Bel, was a Canaanite fertility god.