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Found this piece of trivia....The Universal make-up department submitted two alien designs for consideration by the studio executives. The design that was rejected was saved and then later used as the Mutant in Universal's This Island Earth
Yes, that's true--at least the head of the Mutant was based on a rejected Xenomorph design. But there were other designs, too; a famous article in Collier's featured a big color photo of a lineup of the Creature, the Mutant and a Xenomorph--but it's not the one used in the movie, and it's not the Mutant head, either. It's a large prop with a transluscent head and two eyes (only one is visible in the Collier's photo) that look like they could project out on stalks. It has no discernible mouth. This head, now the property of director Bill Malone, was used on screen--at the end of the Alfred Hitchcock Hour episode "Sign of Satan." A slender model wearing the large head sneaks up on Sir Alfred.

There were also scads of designs for the Xenomorph that never got past the sketching stage; these have turned up here and there over the years. I've seen several that were drawn with silver ink on coarse black paper, making a very interest effect.
In Bradbury's scri pts, the aliens are never described in detail, although in one an alien hand is described as being lizard-like. His scr ipts have been published in an expensive book.