The Dire Wolf
This was the original big, bad wolf. Dire wolves were about 25% bigger than modern wolves, and they had an enormous range of habitats. They primarily feasted on wild horses and bison, but every now and then, a mammoth or a ground sloth was on the menu too (because they were just that badass). Early humans either domesticated or killed off the wolf's prey, and the likely cause of its extinction was starvation as it competed against our ancestors.