Quote Originally Posted by Bob the Spork
The Horned Rat is not a former Chaos God. The four Chaos Gods are a constant. New ones don't arise, and the old ones don't die off. The Horned Rat could only be a very powerful daemon, though in reality the most powerful daemons are practically gods anyway. It's not really a major point.
The god of the Chaos Dwarves (Hashut) IIRC, is a former Khornate deamon who betrayed his master and atteined godhood.
There's also Malal, the Outcast, a major chaos god whose main aim is to destroy Chaos.

I don't recall their names, but there's also a Chaos God who was introduced in a WFRP novel, and who represents atheism and agnosticism. There's also a former Everchosen who, after gaining deamonhood, tried to attein godhood (that would me it is at least possible).

Chaos God a created by other races feelings and ideas. Although GW fluff primarly stick to the 4 major ones and a few minor gods, I think we can basically say there's potentially a crapload of chaos god. It's just that some of them aren't worshiped, disappeared or are just unknown.