Gargantuan lizards 20 feet in length resembling long-legged crocodiles with short, heavy snouts

Teratosaurs are not properly dinosaurs, but rather simply gargantuan lizards 20 or so feet in length; they resemble long-legged crocodiles with short, heavy snouts. Teratosaurs are fierce predators found in plains areas, where they lie in wait in the tall grass until prey comes near, at which point they spring out and attack, surprising on 1-3 on 1d6. They are swift runners but lack stamina, able to maintain full speed for only about 6 rounds before being obliged to move at half-speed until they can rest.