From 38 to 30 million years ago, great herds of rhinoceros-like herbivores called Megacerops roamed this part of Montana. Also known as Brontotheres, they were massive animals classified as Perissodactyla, with three toes on their front feet and four toes on their back feet. Adults stood 8 feet tall at the shoulder, were around 16 feet in length, and weighed as much as two tons, about as big as African Forest Elephants are today. Megacerops had a branched horn on its snout that may have functioned in mating and defense against predators. The climate then was warm and damp, with lakes surrounded by reeds and other riparian vegetation. Megacerops went extinct about 30 million years ago, and scientists have speculated that climate change, changing vegetation, and the animal's primitive teeth may have contributed to its disappearance. The Lakota Indians found Megacerops fossils uncovered by rainstorms on the Great Plains and believed the animal caused thunderstorms when running over the clouds, calling them Thunder Horses.