It is widely accepted today that the early Indians were responsible for the extinction of the large animals of North America - the amazing Pleistocene mammals - mammoths, camels, etc. that were abundant here during the retreat of the most recent Ice Age. However, now it appears that the Indians had a major impact on all edible species, and that the abundance of animals found by the early European explorers may have been a consequence of the Indian population collapse due to disease. Story about the Indians and California wildlife here.
For really old animals - Spinosaurus. It seems that this guy was the largest carnivorous dinosaur. H/T, Lucianne.