It is different from the other Aliens seen before in a number of ways due to the host it was spawned from (a dog in the theatrical cut, cattle in the assembly cut). It is born completely formed and needs only to grow larger. The adult is quadrupedal, has double jointed hind legs and lacks the dorsal tubes of other varieties. It's skin is also tinted brown-ish red.
It is hinted that the Runner may be lacking the intelligence of human-spawned Aliens. In Alien³, instead of gathering hosts to herald the soon to be birthed Queen, the creature killed each and every one of its victims, sparing none, with exception of Ripley twice (although it seemed to have decided to try and kill her in the last part of the movie). This would suggest that Xenomorphs gain intellectual, as well as physical traits from their host. In the AvP video games, the Runner is called the "scout" and is faster than the others, but less durable and damaging.
Abilities and MannerismsEdit
They can spit acid like a Drone or Praetorian. This ability, while mostly shown as a defensive/offensive mechanism (possible to make up for the reduced strength due to it being a scout rather than a fighting Warrior), can have practical purposes. Because their spit has the same pH as their blood, it's quite possible that Runners "dig" tunnels as a means of either exploration or escape. This makes sense, due to the fact that Runners are scouts, and also weaker than the standard Warrior or even Drone.
The Runner Alien was birthed from Spike, a prison dog that was impregnated by a facehugger. The Runner violently emerges out of the dog, coincidentally during the funeral for Newt and Corporal Hicks who were both killed when the escape pod crashed on Fiorina 161. The Runner took residence in the prison's air ducts while it matures and where it claimed its first victim, a prisoner named Thomas Murphy who was cleaning the air ducts. Murphy hears noises coming from a hole in the wall, and goes to investigate, believing it to be his dog, Spike. He soon realizes it is not his pet pooch but the juvenile Runner. Before he can pull his head out of the hole, the alien spits acid into his face. He stammers backwards in agony and accidentally tumbles into a large spinning fan, killing him instantaneously.
After the Runner had reached its mature state, it begins a killing spree by killing prisoners Boggs and Rains while being witnessed by fellow prisoner Golic which caused him to be obsessed with the creature and dubbed it the "Dragon." The Runner then went the the prison's infirmary and killed Clemens, it confronts Ellen Ripley but spares her as it sensed the Queen chestburster within her. It later causes further pandemonium as it killed warden Andrews in front of everyone in the mess hall. Ripley rallies the inmates and proposes they pour highly flammable toxic waste, which is stored at the facility, into the ventilation system and ignite it to flush out the creature. An explosion is caused by the Runner's premature intervention, resulting in several deaths.
The remaining survivors later form a plan to lure it into the foundry's molding facility and drown it in molten lead. The bait-and-chase style plan results in the death of Dillon and all the remaining prisoners, except Morse, who pours the lead. The Alien, covered in molten metal, escapes the mold and is killed by Ripley when she turns on fire sprinklers and sprays the beast with water, causing its exoskeleton to cool rapidly and shatter via thermal shock.