The Runner,[1] also known as the Dog Alien, Ox Alien or Scout,[2] is an adult form of the species Xenomorph XX121 spawned from a quadrupedal host animal (such as a dog). Runners, as their name implies, are fast and agile, and can spit acid from their mouths. They are considered "stage IV" Xenomorphs and molt into Sentries once they've secured a hive and a regular source of hosts. Runners have variously been portrayed as being slightly larger and slightly smaller than typical human-spawned Aliens, differences that no doubt derive from the animal that hosted the creature. However, the Runner encountered on Fiorina 161 was around 7 feet 5 inches tall when standing on its hind legs.