Chaos beasts might hail from Limbo, the edge of reality, other dimensions, deep underground, dreams, or planar storms—anywhere the rules and shapes of existence break down.
A transmuter has found a spell that can lock a chaos beast in one form. This angers some primordial entity of chaos, who sends a plague of the creatures across the land in pursuit of the wizard.
A ranger is so accomplished he has blazed trails from plane to plane. But his last journey took him too near the borders of what is, and a great chaos beast reduced him to an amorphous blob just before as his party was bearing him away through a gate. Now a band of azatas needs his expertise to sneak them across the borders of the Jotunheim of myth, but first a cure for his condition must be found.
A trip through a magic mirror reveals what at first appears to be a mirror image dimension. But any inhabits met there are actually chaos beasts in disguise, who revert to their true forms (or lack thereof) when no longer reflecting into the real world.
—Pathfinder Bestiary 2 54