Ultimate Life Norns
The Ultimate Life Norn is immortal and immune to all blows, bacteria, and poisons. They grow to adulthood as quickly as fast-aging norns, but to keep them from overpopulating your ship, they have been made infertile.
It's a good thing, too, that they're infertile, because they are extremely friendly. They also like to slap each other around to an extent that is probably not healthy for non-indestructible creatures. If you can train them to have good manners, an Ultimate Life Norn in your ship can make an excellent tutor and advice-giver for many generations of "mortal" norns.
The Ultimate Life Norns were created by Edash, and may be downloaded from Edash's Creatures.