They do make good monsters, since they look so alien. They are also really smart.
This particular monster is from Warlords of Atlantis (1978) and it;s a full sized prop and a smaller model. It is very effectively used in a great destruction scene toward the end of the movie. It's prominently featured on the poster, so when I saw the film in the theater I knew I'd be getting at least one cool monster. The rest of the movie is also a lot of fun.
The movie was released on DVD in the UK but not here in the US. It's OOP now, and people want ridiculous prices for it. Perhaps with At the Earth's Core (1976) getting a Blu-Ray release, this won't be far behind.