Actually, what you saw in lots of old sci-fi films were iguanas or other contemporary reptiles with plastic prosthetics scotch-taped to their backs, heh-heh!
Thanks, but I knew they used live lizards with miniature trees, landforms and such as backdrops to make them appear huge.
In fact, I can think of at least two episodes from the 1960s TV Sci-Fi series Lost In Space where they used a live lizard with a Dimetrodon-like ‘ribbed sail’ attached to its back. Lol!
And in the 1960s film The Lost World, among many other films of the time.
But my point was that Dimetrodon was often the creature they were trying to depict in these old films, inaccurately as a dinosaur, when, as the article posted here points out, it, and others of the period, weren’t, dinosaurs not having emerged until millions of years later.
They also used animation, puppets, and/or “go-motion”, whatever exactly that is, in some of these old dinosaur and big monster films.
That's called stop-motion animation.
https://en.wikipedia.org/wiki/Stop_motion
Regards,