The impostor Admiral Leyton was a Changeling and Founder of the Dominion. In 2372, this Changeling temporarily replaced the Human Starfleet officer known by the same name, and was one of four that had infiltrated Earth.
In this form in 2372, he was speaking with Commander Benteen when Odo, in the form of a seagull, landed nearby and returned to his humanoid form. While Benteen was clearly impressed by this, this Changeling's exhibited a certain lack of tact towards Odo, commenting that Odo's choice of imitating birds would have been a waste of time to the Changelings, but he expected no less from Odo due to his "lack of skill when it [came] to mimicking Humans."
Odo reveals Leyton
Odo, who sensed Leyton's hostility towards him, extended his hand to shake with Leyton, and thanked him for allowing him to assist Starfleet, despite their history of mistrust. When he acknowledged Odo's statement, but refused to take his hand, Odo grabbed him, and they linked, exposing the Changeling. After he smiled and said, "Well done, Odo." he shook Odo to the ground; he ran off as he morphed into a seagull and flew away.
Later, the real Leyton questioned why he was being imitated, which lead Benteen to surmise that it was because he had access to all of the various security procedures and protocols, concluding, "The bottom line is a Changeling infiltrated the grounds of Starfleet Headquarters, imitating the admiral, and got away scot-free. Our security measures aren't working." (DS9: "Homefront")
Leyton was played by actor Robert Foxworth.