It is not very difficult to bring dinosaurs back to life anymore, at least in movies. Film special effects have progressed to the point where virtual dinosaurs can easily be built to spec on the computer and directed with precision. If anything, sometimes it seems like there are too many computer-generated dinosaurs stomping around the place.
A few decades ago, though, dinosaurs had to be made from scratch. The most popular way of doing this was through stop-motion animation in which miniature dinosaur sculptures were photographed, moved a tiny bit, photographed again, and so on until an entire sequence could be stitched together. While not always very realistic, it resulted in some very impressive special effects work for the time. But not all filmmakers had the time or budget to fiddle around with dinosaur models.
In the wretched 1960 remake of The Lost World, for example, the filmmakers decided to create dinosaurs by gluing plastic horns, frills, and sails to iguanas, monitor lizards and caimans. The result was absolutely awful. The creatures did not look anything like dinosaurs at all, but even worse was the fact that these poor reptiles were forced to fight each other. Even if television seems crowded with CGI dinosaurs these days at least it has ended the abuse of live reptiles often used in the low budget films of the past.