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:

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 were they just made for the movie, never appearing anywhere else?