A Judas goat is an animal, such as a goat, used in stockyards to lead other sheep to slaughter, while the Judas goat itself is allowed to escape at the last moment. When applied to Humans, the term refers to one who leads someone else into trouble, and manages to keep themselves free of harm.
When convinced by James T. Kirk and Spock to draw the Companion out into the open so that it could be neutralized, Zefram Cochrane recalled the term, as he regretfully knew that he was leading the Companion into a trap. (TOS: "Metamorphosis")
Background
Cochrane's bitter and sardonic reference to himself as a "Judas goat" for his betrayal of the Companion was almost the exact line heard in episodes of television shows in which actor Glenn Corbett was either a regular or a guest star. In the 1968 segment of Land of the Giants entitled "Weird World" in which Corbett guested, series star Kurt Kasnar delivers the line. And in a 1963 episode of Route 66 (a series in which Corbett later became a regular performer) titled "Somehow It Gets To Be Tomorrow", his co-star Martin Milner compares himself to a "Judas sheep... the one who leads the others into the chute."