Isn't some of this just the difficulty of describing something unique?
If you asked me to describe what the aliens from the Alien/Aliens movies looked like, I'd completely fail. If you asked me to describe the alien from Predator, on the other hand, it'd be fairly easy. The Predator is a guy in a costume so I could just describe the differences.
Perhaps the recent Alien movies use CGI or something, but the original Alien from Alien (in the adult form) was the Nigerian actor Bolaji Badejo in a suit.
If you asked me to describe what the aliens from the Alien/Aliens movies looked like, I'd completely fail. If you asked me to describe the alien from Predator, on the other hand, it'd be fairly easy. The Predator is a guy in a costume so I could just describe the differences.