Whatever it is, it appears to only be present in carnivores or omnivores. No Leaf Eater in LBT has it. I don't think Egg Stealers have them either.
I think we've only seen Velociraptor, T-Rex, and Oviraptor have them.
Note: It does seem to be a dewclaw, as I looked at the LBT wiki and both T-Rex and Velociraptors have them, as they do in LBT. (I found this out by typing in the term in the wiki. I wouldn't have known otherwise.)
However, what they are for is a mystery. They can be used for balance or for getting prey.
And that still leaves it open, as all three types that have them DO eat meat in LBT (though Oviraptor also eats plants.)
On the other hand, they could be used for balance too, especially for Ruby.