Birds
Birds are animals which stand on two legs with the exception of the flamingo, which finds it more comfortable to stand on one leg when in a resting position.
Kangaroos
Kangaroos, though they have four legs, routinely stand on two. In occasional instances, these marsupials will drop down on four legs when eating, but the majority of their time is spent standing on their two hind legs.
Primates
Primates such as gorillas, chimpanzees, and monkeys are able to stand on their two rear legs. When walking or running, though, they use their front legs to propel themselves.
Dinosaurs
Dinosaurs are no longer among us, but the Tyranasaurus Rex and Velociraptor were two who roamed the earth on their two rear legs.