From Creatures Wikia
The Zombie Norns are a green tinted undead breed created by Empyreal. They lack any chemical for life, and yet continue to live, thus undead. They are extremely succeptible to diseases, which decay them. They do not age, but can be injected with a growth hormone, chemical 14, to make them age to adult. As babies, they have an insatiable appetite to fuel the growth hormone, and as adults are eternally friendly. Despite being so friendly, they never seem to overpopulate the world. Babies will never die of old age, but once they are injected with chemical 14, they should die in about an hour. They also have almost no need to sleep, but occasionally will rest.