My theory here is that the primal gods of the animal-humanoid races were in bad shape, like the World Serpent of the Sarrukh. Not only that, but demons had found the created races easy to suborn.
1.) Gnolls seak wisdom from Lizardfolk on how they've avoided/defeated Sess'Innek.
2.) A group of Gnolls loses their Flind, and finds they can think clearer and seeks aid.
3.) Demons attack a village/town, and Gnolls come to the rescue? What?
Their creator god is unknown; a lesser god named Gorellik taught them to hunt and tame hyenas, but offered no divine magic of note. Gorellik was defeated by Yeenoghu. Gnolls have been known to turn (successfully) to the gods of the Giants.