Vipers are a mutated subspecies of Dragon corrupted by Menonias' dark magic. They lost their wings and hind legs in the process. The glands that secreted the fire-breathing fluids turned to venom sacs; the caustic substance is corrosive and incredibly dangerous and - like with most things corrupted by dark magic - have the ability of corrupting others of their kind, like Vampires do with the living.
Ith was the first Viper and was capable of controlling all others through sheer will alone; when Menonias first turned him, he began corrupting other Dragons. By the time of "Book 1: The Heirs of Menonias", only a handful of the monsters still live, many hunted and killed by the Dragonmyst Ith tried to kill for decades.