Dagarah72 mentioned lizards with rubber fins stuck onto them. That's a entire subject worth discussing itself.
I'm sure I'm missing several movies, but didn't that start with Hal Roach's
One Million BC in 1940....?
We all know why they used this method..... a cheaper and quicker alternative to stop-motion, and used when a man-in-suit (badly done ones, at that) wasn't.
King Dinosaur, another Bert I. Gordon baddie, from 1955...
http://www.youtube.com/watch?v=Ilm_VgYUmcM
Journey to the Center of the Earth (1959) - maybe the best use of lizards as dinosaurs, they were adorned with a spinal fin to look like a herd of Dimetrodons.
...here's a bad copy of that scene, camcorded off of a tv...
http://www.youtube.com/watch?v=zH6HLLDyg2Q
And here is that scene from an Italian dubbed version of the movie, starting at 3:05 minutes in .....
http://www.youtube.com/watch?v=7iV2KSqU9G8
Hard to say exactly how many they used, as they were obviously photographed and edited together in a way that made it seem like there were a lot. And when the explorers kill one of them with spears, you see the others taking hefty bites at its corpse.
There was the drive-in cheapie
The Giant Gila Monster (1959), and I've only ever fast-forwarded through it. It's pretty bad.
Entire movie:
http://www.hulu.com/watch/38082/the-giant-gila-monster
Trailer:
http://www.youtube.com/watch?v=_3b0NWljlPw
The Lost World, the 1960 Irwin Allen version of the Sir Arthur Conan Doyle novel:
Brainless fun, and a HEAVY reliance on lizards for Dinosaurs. Even at the film's end, an egg breaks open, and Professor Challenger loudly proclaims, "A Tyrannosaurus Rex!" when it's a tokay gecko with horns stuck on its head.
A lot of color screencaps here:
http://monstermoviemusic.blogspot.com/2 ... ic-by.html
Hoisted by strings under its forearms out of a bubbling pool, and then stuff dropped on it later....OH, and a little dummy with two legs hanging out of his mouth in a few shots.
Here's their obligatory lizard/alligator fight, and take a good listen to the roaring of the alligator, about 1:33 minutes in. The first time I saw Star Wars in 1977, I knew the sounds of the TIE Fighters sounded familiar!

As the scene drags on, though, these reptiles are truly fighting, and it's kind of ugly to watch, knowing what went into filming it.
http://www.youtube.com/watch?v=d_VwO2tqYCc
Oh, and all that footage was used endlessly in other Irwin Allen productions (tv shows) throughout the 60's, but I've seen the stuff used elsewhere, too....even a throwaway scene in
When Dinosaurs Ruled the Earth.
The last instance I'm aware of a lizard's use was in Hammer's
One Million Years BC in 1966, Ray Harryhausen wrote in his book Film Fantasy Scrapbook something along the lines that he wanted to transition the audience in to the effects of the animated dinosaurs with the use of a live lizard first. That was an innocuous one, compared to the other movies, where the lizard scenes generally got violent with animals being made to fight each other, having things dropped or spilled on them, get thrown over cliffs, and all manner of fins, horns, spines, frills attached to them....
As a wee kid, they enthralled, and it truly felt like we were seeing huge prehistoric monsters, but Mom and Dad were always right there to tell us how they did it
...and looking back on it, it's definitely campy, and, well....kind of nasty.
I've had reptiles as pets before, and I don't think you have to be a stereotypical PETA card carrying member to see that this is just mean. After putting this post together, I'm feeling a little queasy.
I suppose what I want to know other than "what were they thinking?!" , is
how they did it.... how those prosthetics were attached, how they stayed on during shooting, what they did to goad the animals into fighting.... what they did with them afterwards. As I understand it, the SPCA DID put their foot down on this, albeit 20 years after the first instance of it. My guess is that the scenes in The Lost World were the turning point for this.
There just doesn't seem to be much on record of how they did this stuff, behind the scenes footage or pictures, etc..... almost like they were ashamed or knew that it wasn't right.
So what other movies have I missed any other movies that used these kind of effects, or can anyone share some insight on them?