Alien: Chestburster (Retired)

Its egg is laid in the host's bronchial tubes by a Facehugger, where it hatches. It then burrows for the host's aorta, where it feeds on absorbed nutrients and white blood cells. Until it finishes feeding on the aorta, its life is dependent on the survival of its host, but after it finishes, it will not require the host to survive where a normal chestburster emerged from a victim even after he had been shot in the head and the Predalien hybrid emerged from a dead Predator, which had been alive long enough for the chestburster to finish feeding. After about an hour or two, it lets go, and begins swimming about in the host's body, causing mild discomfort. Then, it finds the sternum, weakens the structure with acidic saliva and starts chewing. This causes the victim great pain, shock, and blood loss. Then, true to its namesake, it pops out, and finds a nursery or chicken coop, or, less messy, a food locker, something with a large amount of helpless meat. Its carapace has yet to harden, and is a dull grey fleshy hide. Depending on the host, it will grow to become a Drone, Predalien, or Runner. Runners have all limbs, tail and inner jaw fully formed when they emerge. Genetic alteration causes it to split into identical twins after escape from a human or quadruped.
An embryo implanted in a pregnant woman will feed directly on the baby, and will erupt from the stomach of the woman.