These things are the stuff of nightmares. Appearance-wise, that is. Mechanics-wise, they aren't that great.
I used to think chaos beasts were worthless. After carefully reading their statblock, I've changed my opinion: they're only mostly worthless, with the lack of worthlessness coming from an ability that is unlikely to actually change fights.
Allow me to explain. Corporeal Instability is a decent save-or-suck (no spellcasting, severely lowered speed, hampered attacks). If it were just that, it'd be outclassed by many other abilities, but the thing is that CI is incredibly hard to escape from. It doesn't naturally wear off, a charisma check only suppresses it for a single minute, and the handful of spells that do remove it are high-level or require expensive material components. Also, if the continuous wisdom drain knocks someone out, they turn into a chaos beast themselves.
Of course, because a chaos beast still has only two weak claws for natural attacks, can't use equipment, and can't really contribute to a fight against nonliving or transformation-immune creatures, it's still a bad choice. A poor save-or-die that only kicks in a couple of minutes after the battle should have ended and spawns a hostile monster when it finally kills the target isn't a reason to forgo eight levels. -0 LA works.
I also wish to apologize for the lack of updates lately. Things have been busy recently, but at the moment it looks like I'll be able to update more frequently in the nearby future. I hope everyone understands.