Paraceratherium
A lot of the animals we find ourselves wishing for are bigger versions of something similar that is still living. Some are just big. Paraceratherium is probably the biggest land mammal that ever lived. So obviously we want to see some of those, walking around, dwarfing elephants (and also the mammoths we also brought back).
We strayed into the millions of years ago territory we said we'd avoid to include this one, but come on. These crazy looking things were around 16 feet tall at the shoulder! They were almost 30 feet long and probably weighed 18 tons. Whoa.
Images: Top: Wikimedia commons. Right: Wikimedia commons.