Changelings are a race of gelatinous non-humanoid species native to the Gamma Quadrant, close to the Omarion Nebula.
Little is known about their origins other than that they had once been an exploratory race which had been treated with mistrust for so long that they changed their priorities to conquest. Thus they formed the Dominion.
In the 23rd century, before initial first contact, Travis M. Hatcher met his descendant from the future who had come seeking a Changeling he had referred to as the Only. During their search, they discovered much about the history of the Founders.
Apparently, they were from a relatively young race of proto-humanoids fathered by the "Progenitor", which turned out to be an insane human of the distant past. From early on, this race had inherited their progenitor's knack for deception, creating messages using ancient technology where they claimed to be an ancient race which had started the evolution of all sentient races across the galaxy. As they began exploring the galaxy, they encountered various hostile races, which caused them to reconsider their goals from deceiving the races into worshiping them to genocide. They therefore created the Dominion to act as their arm and fist and a prison-realm as a means of slow-and-orderly disposal of the conquered races.