Changeling

There are many fey with the ability to change shape, the changelings are simply the most common, they are a fey creature that have a basic form that looks thin, lanky in a roughly human shape, completely white, they have straight hair, in this form, they are not biologically distinct male or female, and have no genitalia at all. They hold no size limits, changelings are known to decently often steal children to take away to the fey realm, as creatures of identity they require a creature that has yet to make one in order to create a changeling. and so in order to create a changeling they require a child that has yet to decide who they are in life, this can theoretically be any age, but the younger, the less likely they are to have enough identity to be unviable. this child is then raised by the changelings, until it's part of their family, then 1 night, they shed their body become a changeling, their body staying behind in a husk. from there a changeling can gain a identity as they are changeling, there are those that later in life decide what they did was wrong and go back to their parents.
Replacing children is also a tactic that changelings sometimes employ to gain a footing in the family, or to make sure they won’t hunt after their missing child. This is seperate from stealing the child often, though myths say they have to do with each other.