The mosquito on four legs thing? Yeah it's there, I guess it probably was going to be part of the fauna of LV-1201. But the Rig is extremely simple and the animations are down to a few. It's likely they started to work on this slowly and eventually they shifted their focus or didn't have time to implement more stuff in the model.
There are a lot of things like this that got unused like the Runner animation in water, I guess it was planned for each species but never implemented in the end because water play a so little part.
Primal Hunt even have some "accident" in the Rez file, some models aren't smooth shaded and they completely forgot to remove it from the package resulting in two models but with one have hard edges which isn't suitable for the game to display properly. The most notable is the Bull thing. Its animations are so primitive and the model is doubled, it's like they packed the thing in the last minute to sell it.
On the Predator Campaign mission Old Debts shortly after you kill the two predbugs and a few more groups of aliens and go through a dark cave if you go near the skeleton of whatever that creature is you can hear what seems to be growling and occasionally the ground shaking! Was that an unused enemy that never made it to the game? Is it the same creature as that skeleton? I am not the biggest AVP 2 fan but it was always the stuff that was only touched upon or hinted at that intrigued me most.