Giant lizards are one of numerous mounts that the nomads of Prax ride. These are among the less intelligent beasts, and while they may not be as intelligent as other creatures, they are durable.
Equal opportunity eaters, they are as comfortable eating carrion as well as eating fresh meat. As such, their vicious jaws afford them an extra mode of attack that others who ride herbivores don't have.
Note: It is certainly possible these may refer to a type of dinosaur as well, but it isn't made clear.