From Creatures Wikia
The Cave Norn is a Norn in Creatures Playground, which can be unlocked by finding all of the bones on the playground. However, there is only one male Cave Norn, classified as a Grendel, making it impossible to have more than one Cave Norn in the game. The name of the Cave Norn is Urrg (Source).
Norngirl patched the Cave Norn, adding female sprites and making him a normal Norn hatching from the original eggs. However, you have to overwrite one of the original genomes to install the patched Cave Norns, resulting in the loss of the particular original norn breed.