A well-known easter egg from Predator 2 is the presence of an Alien skull in the trophy case on board the Predator ship. Aside from a couple of human skulls, there are 4 others:

And here's a close-up of the two skulls abutting the Alien skull:

Can any of these skulls be identified as either homages to other sci-fi creatures (like the Alien skull) or belonging to creatures appearing in any of the subsequent Predator comics, books, or other movies? Or are they simply props appearing in just this one movie?