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. His physical appearance is not described much 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". To some, 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. An argument against this theory is the fact that Dandelion says that no one knew what sort of animal they were and they had not been seen by any living rabbit, suggesting a mythical creature, as the rabbits certainly mention hares during the course of the novel more than once as living creatures and allies.