Dukan'Rex was a Jem'Hadar.
In 2371, Quark purchased salvage from the Gamma Quadrant from the Boslic freighter captain Rionoj. When Quark examined it he found a baby inside a stasis chamber. At first, the Deep Space 9 crew did not realize that they had found a Jem'Hadar, until a the child rapidly developed due to inherent genetic engineering. As Odo, a Founder, a god to the Jem'Hadar, was the only person the young Jem'Hadar responded to, Odo tried to break free the youth of his violent nature. Odo came to see there was little hope of doing so, especially when the youth threatened to kill anyone who attempted to stop him from returning to the Dominion. Odo soon returned the Jem'Hadar to the Gamma Quadrant. (DS9 episode: "The Abandoned")
The Jem'Hadar later took the name Dukan'Rex and would rise to the rank of First, and become the leader of the Jem'Hadar in Odo's fleet during the conflict with the Hur'q in 2410. (STO video game: Victory Is Life)