Origin: Gamma Quadrant
Little is known about this strange race. They call themselves Changelings, a name adopted from others' use. The Changelings are the highest order in a Gamma Quadrant faction known as the Dominion which is composed of at least four distinct species. Within the Dominion, the Changelings are known as "The Founders" and are greatly feared.
These gelatinous beings exist in a great link, combining in totality to form a large liquified area resembling a lake where they share a collective memory. No Changeling has ever been known to harm another.
Constable Odo on space station Deep Space Nine is a Changeling, having been found by a Cardassian scientist. Odo is able to morph into many shaped, regardless of the total perceived mass of the object.
According to the link's collective memory, the Changelings once roamed the stars in peaceful exploration and contact but were feared by "Solids" and met with hatred, suspicion and violence — hunted, beaten, killed — until taking refuge on the secluded homeworld they now inhabit. Although their physiological development is unclear, memory is not a trait of the "newly formed."
They once sent out Constable Odo and 99 other such "infants" to satisfy their latent quest to explore, genetically implanting each one with the desire to return home. Constable Odo is the first to do so, thanks to the Bajoran wormhole. They were not expected back for another 300 years, and his female mentor says only that he was away "a long time."
They have no sense of smell. Scans of a morphed Changeling merely read as the object being modeled would normally. The best test of a Changeling is through a simple blood sample; once separated from the body, a small amount will quickly turn back to its natural state.