Onagers are wild asses once found throughout the Middle East. Along with the takhi horse of Mongolia and surrounding regions, they always represent to me the essence of the primitive horse (or more properly primitive equine) that has always occupied the corners of my mind. Buff colored, with pale snouts and stripes, these are the animals from which the modern horse and donkey were bred. The modern onager itself can be tamed and ridden, and they may have been domesticated in ancient Persia, where they appear in art pulling chariots.
Using these primitives instead of modern equines in an otherwise contemporary story adds a touch of the mysterious, a whiff of alternate history. Or similar animals might be tamed and eventually domesticated by colonists in a science fiction setting for an oddly familiar echo of Earth, just slightly out of phase.
Look at this head, so ancient and wise, and yet filled with good humor, and possibly a touch of the rascal. This creature would make a great character in a comic, either as a wild animal or as a domesticated (or partially tamed) companion to a human or humanoid, past, present, or future. I took a lot of photographs of this guy.
Read more about the Persian onager and the efforts to preserve it here.