Changeling Infant

Star Trek: Deep Space Nine
Episode: DS9 510 - Begotten

Nameless young Founder that was found on Deep Space 9 in 2373. Fellow changeling Odo took the young lifeform under his wing and oversaw its early development. When the young changeling was about to die, due to overexposure from tetryon radiation, it managed to infuse itself into Odo, who in turn regained his shapeshifting abilities.