Well, were giant octopus, were squid, and were giant squid also work.
The scorpion's grasp feat (Sandstorm) has the effect you're looking for. It's not explicitly improved grab, if you need it for a prerequisite.
You still have to make the initial grapple check, and it still automatically fails if the creature is two or more size categories larger than you. The only thing I know of that gets around that is mimic.