A hindlimb or back limb is one of the paired articulated appendages (limbs) attached on the caudal (posterior) end of a terrestrial tetrapod vertebrate's torso.
[1] With reference to quadrupeds, the term hindleg or back leg is often used instead.
In bipedal animals with an upright posture (e.g. humans and some primates), the term lower limb is often used.
The main reason is it can jump high to easily escape to its predator and also to catch prey.
The first vertebrate bipeds were the Bolosaurids, a group of prehistoric reptiles with no living relatives.