The Alien Runner, or just "Runner," was created as the primary antagonist for the third Alien film, with its singular presence linking back to the similar scenario in the first Alien film. The Xenomorph for the third film went through multiple revisions, both conceptually and as an actual stage creature. One of the earlier ideas was to have a trained dog wear a Xenomorph costume and act as the Runner in the film, but this idea was soon scrapped. Deleted scenes also show that instead of having spawned from a dog (giving the Runner the popular nickname of "Dog Alien"), it was originally meant to have spawned from the body of a dead ox (giving it a less common nickname of "Ox Alien"). The gangly appearance of the four-legged chestburster that erupted from the dead ox also earned it the nickname of "Bambi Burster."
Powers & Abilities
Alien Runners have many of the standard Xenomorph characteristics - they possess the same razor-sharp claws, fangs, and tail, as well as acidic blood and the prehensile tongue mouth. Runners are quite a bit smaller than most other Xenomorphs like the Warrior though, and it furthermore lacks the dorsal tubing on its back (most likely to give its head more maneuverability). Being less hardy than Warriors, Runners make use of their greater speed and agility to outmaneuver enemies instead of attacking them head-on.
Runners also have the additional ability to spit acid at enemies and tend to be colored a brownish-red rather than the standard black, although this coloration may simply be a camouflage response to its environment, as the Runner in Alien 3 was able to hide itself much more easily than a fully black Warrior might've been able to (although this coloration seems to have been overlooked in the Alien 3 comic book adaptation).