Changeling

A changeling is a creature found in Western European folklore and folk religion. It is typically described as being the offspring of a fairy, troll, elf or other legendary creature that has been secretly left in the place of a human child. Sometimes the term is also used to refer to the child who was taken. The apparent changeling could also be a stock or fetch, an enchanted piece of wood that would soon appear to grow sick and die. The theme of the swapped child is common among medieval literature and reflects concern over infants afflicted by as-then unknown diseases, disorders, or developmental disabilities.

Deep Space Nine
The Changelings were liquid-based, shapeshifting lifeforms native to the Gamma Quadrant of the Milky Way Galaxy. Their homeworld was, apparently, a rogue planet within the Omarion Nebula (which has since been rendered uninhabitable by the Obsidian Order and the Tal Shiar in the Battle of the Omarion Nebula in 2371).

The Founders were at least partially composed of morphogenic enzymes, the molecules responsible for their shapeshifting ability. In its natural state, a Changeling's body was a formless gelatinous mass, to which it had to revert every sixteen hours (Odo's example) in order to regenerate. If a Changeling remained in solid form for longer than sixteen hours, their body began to deteriorate and peel or "flake' away.