Although computer-generated images are being used more and more to give life to cinematic creations, mechanical models tend to be less expensive if a creature has to appear in a relatively large number of shots.
Movies often use both.
The giant predatory title creature of Anaconda was computer-generated for about 20 percent of the shots and models for the rest. Calif. Creating a robot that mimics the movements of a living creature realistically is difficult under the best of circumstances. Furthermore, this equipment had to work consistently and reliably as the snake whipped around and attacked its victims. Since some scenes were underwater, the snake also had to be waterproof. The larger snake weighed 5,500 pounds and operated at peak at 600 horsepower.