Like many other large grazing animals, its range has contracted greatly under the pressures of hunting and habitat loss, and of the four subspecies, one is extinct and two endangered.
Onagers are a little larger than Donkeys at about 290 kilos and 2.1 metres (head-body length), and are a little more horse-like. Some authorities regard one of the four races as a separate species.
The onager consisted of a frame placed on the ground to which a vertical frame of solid timber was rigidly fixed at its front end; through the vertical frame ran an axle, which had a single stout spoke. On the extremity of the spoke was a cup to receive the projectile.
In action the spoke was forced down, against the tension of twisted ropes or other springs, by a windlass, and then suddenly released. The spoke thus kicked the crosspiece of the vertical frame, and the projectile at its extreme end was shot forward.