Paraceratherium
Paraceratherium is probably the biggest land mammal that ever lived (an Indricothere Hyracodont from the Oligocene, 23 - 33 mya). 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: Dmitry Bogdanov - Wikimedia commons. Right: AMNH - Wikimedia commons.