The Shadow Norns are really special, they look like a shadow! If you put a Shadow Norn in the Grendel Jungle, it looks like the Jungle; if you put it in the Norn Meso it looks like the Meso, as they are transparent. They are also amphibious, and tend not to wall-bonk so much.
The Shadow Norns were made by Marcus K. quite some time ago. Marcus forgot about them, but remembered when Tina asked for a norn breed for her website. Now you can download them at Tina's Creatures Paradise.
If you breed the Shadow Norns with normal Norns, the offspring will look very funny. ;-) This picture taken by Marcus on this site shows the head of a Shadow Norn mix.