The Harlequin Norns were an additional breed released by Creature Labs in conjunction with the Docking Station Story. You could previously buy the Harlequin Norns at the Gameware Creatures Mall, but they are now available as a free download from Creature Labs. They are also included with Creatures Exodus.
The creation of the Harlequin Norns is discussed in the Harlequin Norn Story.
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Harlequin Norns were created by the Lone Shee - they are basically just ChiChi Norns with amusing fur patterns. However, they got laughed at by the Magma Norns, so the Lone Shee sent them out into the Warp to find a place of their own.