When the first people came to North America they found a land populated with many animals we wouldn’t recognize today. These weren’t dinosaurs (remember, they died out 65 million years ago) but were animals like mastodons, wooly mammoths, saber-tooth cats, giant ground sloths, short-faced bear, big-horned bison, the American lion, horses, oxen, and camels. Nearly all the large mammals of Ice Age North America became extinct in the space of 1,000 years, perhaps due to massive climate change and the arrival of people. Horses were reintroduced to the New World by the Spanish.