Didn't they used to use giant iguana shots as dinosaurs? I seem to recall that at least Flash Gordon (Buster Crabbe) fought one
They did used to do that in "Flash Gordon", and also in some 50s US monster movies. I think one was "The Giant Gila Monster".
Oh, yeah, Gecko-Roman Wrestling. There was an MST3K ep that featured that, either first or second season. Can't remember which one it was, though. Corman or Lippert film, I think.
MST3K never did "Them!", but they did do "Phase IV", in Season Zero.