Roddenberry's original concept for the Talosians was as "crab creatures". In February 1964, The Outer Limits did an episode called "The Invisibles", with alien creatures resembling crabs that were forcibly placed on men's backs to turn them into mind-controlled slaves. I've always suspected he might have been thinking of using those same creature models for The Cage.
The crab creature:
There was a rather intense scene in The Invisibles where actor Don Gordon (another good actor who really should have done Star Trek) was being subjected to the procedure. I remember one publication used a still of this scene and misidentified Gordon as Burt Reynolds, who isn't even in the series.
I always liked Outer Limits' monsters over Trek's aliens.