Changeling

Changelings are shapeshifting humanoids, not quite elves but not quite human.

Description
Changelings are the result of, well, a mistake. Basically, Elves don’t reproduce. They technically can, but due to their origins every successive generation becomes more and more mutated, and so don’t. Most Half Elves and High Elves are actually the product of a specific spell performed on kidnapped human children. But before the process can actually be performed, the human must be prepared. They are transformed first into a Changeling, a creature capable of altering their very physiology, so that they may more readily receive the treatment. But sometimes... the process fails to take, and a Changeling is left in their raw shape. The existence of Changelings is a closely guarded and embarrassing secret of the Elf kingdoms, for one to be out adventuring is an extremely rare occurrence.