King Darzin is a character from the myths of El-ahrairah. He ruled over the biggest and richest of the animal cities in the world at that time. He is not describe at all but he is clearly not a human.
Rabscuttle was able to hide in plain sight among the children of Darzin's kingdom, "as he was about the same size as the children and not much different to look at". It seems likely then that King Darzin is a Chief Hare and his 'animal city' is a colony of hares - who compete with rabbits for many of the same resources.