Are you thinking of the hand from EVIL DEAD II? The poor guy's right hand was possessed, attacked him (as did Dr. Strangelove's) & unlike the doc, he chopped it off. It then crawled across the floor, climbed up the tablecloth, etc., on its way to get him.The hand, the detached hand from Hands of Orlac, and there was another hand that crawled around in some movie.
In another version of Fredrick Brown's Arena, this one in the Outer Limits, these creatures as I recall, were not even identified. Except for the silliness factor in the Star Trek version, Fun and Games is my preference, because it is really creepy.The Gorn from the Star Trek (TOS) episode Arena. I was a kid at the time and thought the guy in the ill-fitting rubber suit must be awfully hot. Suspension of disbelief kicked in and I sat back and enjoyed the show. How will Kirk get out of this one?
Big screen: The Mist, though technically not a monster. Watching that movie made me terrified of foggy days, and every silhouette I spot through them. Small screen: Weeping Angels, because how can you not love that concept?