"Judas goat" is the official slaughterhouse
term for an animal used to lead others down the ramp to their death
. Outside the abattoir
environment, however, the phrase most often finds use as an insult (particularly in political debates
, for some reason) referring to those accused of leading others into trouble while keeping themselves free from harm.
Judas goats are primarily used to slaughter sheep, who can not be readily driven to their death, especially in modern slaughterhouses where they may have been trucked beforehand for as long as three days, wide-awake and without food or water. The slaughterhouse workers thus take advantage of the herd instinct--a goat acting in alpha male fashion will be readily followed by the sheep, even on to the killing floor.
Beef slaughterhouses make use of "judas cows" (as cows won't generally follow a goat) and the term is still occasionally used to refer to the shills at crooked carnival games who attempt to convince observers that the game is not only winnable, but easily so.
No, there are no judas priests.