Predator 2

A well-known easter egg from Predator 2 is the presence of Alien skull in trophy case on the Predator ship. Aside from a couple human skulls, there are 4 others. Probably to show  that Predators are not just predators of men, but whole galaxy. They travel the stars looking for "great hunt". It was also a nod to the Dark Horse Aliens vs. Predator comics wich were quite popular at the time. Idea came from Stephen Hopkins. Since Fox had own the Alien film series, it was easy  to obtain the rights  to use the Alien head in the film. This excelled popularity of the Aliens vs. Predator series crossover troughout the 1990s, and was promptly followed by more comic books, novels, video games, toys and eventually movies.