It's funny actually.. when looking at the main creature sketch, the "Relic" creature just popped in my head and what an awesome design that was.. though the film kinda sucked. The Relic creature was a mix of I think..iguana, insect and human or something of that nature. I just imagine this thing running and pouncing with the ease of a rabbit/frog or in the same motion of a kangaroo - hop, bounce, run, pounce vestigial limbs/spikes spread and the large opposable digit stretched outright as claws and spiked limbs tumble down upon helpless prey. I imagine it would use all its clawed limbs and digits to rip small chunks of flesh off and fed into the mouth with the beetle-like appendages near the mandible.
If you've ever caught a frog, toad or salamander/newt. Imagine how creepy it would be if you knew they had sharp teeth and claws. It would be an odd mix, in contrast to the soft skinned bodies and the usual soft padded toes and webbed feet.