Auriel

Only the pure may ride it, so it never has been.

Forms

The same creature at each step of its evolution. A form is crafted from the one before it, so the ladder is climbed in order.

This creature has a shiny version at every step of the ladder. Shiny is its own roll, so it can turn up on a plain form or on a fully evolved one.

Auriel hatches from the Eternal Kingdom egg. Every egg you open in that world can give you one, and the rarer the creature the longer that takes.