Spoiler Alert! Dr. Terence Wynn was the "Man in Black" wearing cowboy boots in the 1989 movie, Halloween 5: The Revenge of Michael Myers. On a rampage, he killed a lot of cops and broke Michael out of prison.
Dr. Wynn's identity is not revealed until the 1996 film, Halloween: The Curse of Michael Myers. He wanted Jamie Lloyd's baby, Stephen, to be the final sacrifice for Michael and the Curse of Thorn would then pass to Danny Strode. Unfortunately, he was supposedly killed by Michael with a machete before he got the chance.
Halloween 5: The Revenge of Michael Myers is considered to have one of the top ten worst horror movie endings.