The first one: Glyptodon. It was basically a big armadillo, but the size of a tiny car. The cool thing is that it actually coexisted with humans, as it went extinct 4000 years ago. There's also fossil remains that show damage from man made spears.
Second one: Terror bird. This thing hunted horses. It went extinct around 2 million years ago. It couldn't compete with the sabred cats that migrated to its continent when north and south America joined.
Third: Elasmotherium. Look at that ridiculous giant horn.
Extra: Paraceratherium. The largest land mammal that ever lived. This animal towered over elephants. It was actually part of the rhinoceros family.
There are so many cool extinct animals to talk about, like the megafauna of Australia (Andrewsarchus, Megalania). An interesting topic to talk about is why so much of the giant animals died out so fast.