Especially in Judas sheep and Judas goat, the name Judas is used as a modifier to designate:
– literally: an animal, especially a sheep or a goat, used to lead others to slaughter;
– figuratively: any person or thing used as a decoy to lure people into being caught, arrested, etc.

The allusion is to Judas Iscariot, the disciple who betrayed Jesus in exchange for thirty pieces of silver.

 

LITERAL USES

 

The earliest Judas sheep that I have found mentioned—in The New York Times (New York City, N.Y.) of Saturday 24^th March 1900—was named Judas Iscariot:

“JUDAS ISCARIOT”—BUTCHERED
Armour’s Veteran “Leading” Sheep Pays the Penalty at Last.
Special to The New York Times.

CHICAGO, March 23.—After having led thousands of confiding sheep to their death, “Judas Iscariot,” as he is called in the yards of Armour & Co., has paid the penalty of his treachery and been butchered. For eight years “Judas Iscariot” has been the “leading” sheep for the company.
Last week Judas rebelled. He refused to work, and his execution was decided upon. It is said by stockmen that a sudden attachment for a snow-white feminine sheep among the victims is responsible for his rebellion and ultimate death.

The second-earliest mention of a Judas sheep that I have found is from The Chicago Daily Tribune (Chicago, Illinois) of Sunday 19^th February 1905—in this article, goat in “a young and frisky goat” probably designates a sheep [cf. note]:

Knows His Job Is Endangered.
By J. L. Graff.

Does a work animal know when another is in training to take his place?
W. B. Throop, one of the division superintendents of the Burlington road, says that his company owns a sheep that seems to have an intuition that he is to be knocked out of his job, even if he isn’t given a final and finishing thump on the head.
The animal referred to is Dick, the bell wether that for some years has been used at Montgomery and Plano to lead great flocks of sheep out of the cars on to the pastures that are controlled by the railroad in feeding flocks that have been brought in from the west. It has been the duty of Dick to act also the part of a Judas. After he has led his tribe out on the green and nutritious pastures of Kendall county and staid there with them for some days, he is called on to march the same woolly coated creatures to their death. He leads them back to the cattle pens and into the car in which they are taken to Chicago and there slaughtered.
Dick is getting old, and the company fearing that it might be left without such a valuable aid, has hit on the plan of making Dick train his successor, and this he is now doing with all the faithfulness that has characterized his actions as a herder. But the look of anguish that spreads over the face of that sheep is pitiful in the extreme. He seems to know that he is putting in his last licks, that after all of these years of faithful service he is to be supplanted by a young and frisky goat that never did and never will know anything about the business. Now and then Dick will raise his moping head and open his eyes long enough to take a square look at his sheepy understudy. For an instant there is the old fire in the wether’s eye, and one half expects him to make a lunge at the young fellow that will shunt him over the pen fence. But he seems to think better of it, contenting himself with a shake of his head and neck that starts the tinkle that in the past has called thousands of his kind first to a feast and then to their everlasting doom.
Supt. Throop says that the young wether is learning fast and when Dick dies, which he is liable to do at any time, he is to have a good burying box and a marker. Some heartless ones wanted to make mutton of him, but such a thing won’t be allowed.

Note: The word goat has been used to designate a sheep. For example, the following text and photograph are from “Judas” Sheep Lures 750,000 Of Own Kind to Death, an article about a Judas sheep named Billy, published in The Missouri Herald (Hayti, Missouri) of Friday 23^rd April 1926:

Billy has no conscience. […] Of course Billy’s not human. He’s only a goat.

Here’s Billy, the Traitor

The earliest mention of Judas goats that I have found is from The St. Louis Republic (St. Louis, Missouri) of Sunday 2^nd April 1905—the illustrative photograph shows a goat leading a drove of sheep:

Judas Leading the Lambs to Slaughter.

“Judas,” Treacherous Goat, Trained by Packing-House to Lead Sheep Droves to Slaughter in Butchering Pens

The goat family has been cartooned for time immemorial and with the goat has usually been associated one or more old tin cans or a washing hung out to dry. But he has a larger sphere of usefulness and it remained for a large corporation to develope [sic] it and to enable him to come into his own as a trained worker.
At the National Stock Yards in East St. Louis the sheep pens are quite a distance from the packing plants. As there are several open roadways to cross, it required six or seven men to get a drove of sheep over to the butchering pens. Someone conceived the idea of training a goat to act as leader for the sheep when they were sent over.
The experiment proved so successful that now each packing-house has its own goat guide, which knows the route from the pens through the runways and over the viaducts to that plant.
He is named “Judas” because he leads the innocent sheep to slaughter.
As soon as a lot of sheep have been weighed, for which every firm bought them, Judas is turned into the pen and the gate is opened. He looks over the lot and then saunters out of the gate, the sheep following. He goes slowly at first to give them a chance to make up their minds to follow.
They are very timid in this strange place, but seem to think he will lead them to green pastures. But he does not. A man and a shepherd dog follow the drove to keep the stragglers moving.
In ten minutes Judas arrives at the end of their life’s journey with his tourists and side steps into a chute until they have been driven into the house, then he comes out and leisurely returns to the barn.
When Judas becomes old or gets too lazy to do his work satisfactorily another goat is trained and the first one makes a trip from which he fails to return, except in another form, suffering the fate to which he had led so many thousand sheep.

The following unsigned essay was published in many U.S. newspapers in April 1926—for example in The News-Palladium (Benton Harbor, Michigan) of Thursday 8^th:

HUMANS AND SHEEP

If you want a little lesson in sheep nature that is a lesson in human nature as well, go to the sheep pen of a slaughter house.
Down below are hundreds of sheep. They are all alike. There is very little to distinguish one from another. They crowd about in a woolly mass which shifts here and there about the pen. One bleats and they all bleat. The mass moves this way and that.
Then a man leans over to a wise-looking old sheep wearing a bell. The old sheep bleats understandingly and starts walking toward the killing room. The bell tinkles and the sheep follow that sound, follow the bell-wether, their leader.
Up to the very door of the slaughter house the mass moves. Then the man calls to the leader again, and the old bell-wether leaps to one side. He stands watching his fellows pass to their doom. Eagerly the creatures run through the door to their death.
Every day it is repeated. The Judas sheep leads thousands of his fellows to their betrayal. And the thousands always follow, bleating timidly, rushing blindly. But always, on the threshold of death, the leader leaps aside to safety.
If a sheep could only investigate! If one sheep in all those thousands would only stand at the gate of death and turn back his fellows, fight them back to safety.
But sheep-thinking is the thinking of the mass, the mob that runs blindly guided by a straw of instinct. The sheep do not stop to investigate. They hear the tinkle of that bell and follow.
In the world how many are like that!

Other animals have been employed as Judases—as mentioned in Farm Ranch and Garden Department, the agricultural section of The Seattle Daily Times (Seattle, Washington) of Saturday 21^st May 1921:

One of the very necessary features of every packing house is the “Judos,” [sic] an animal trained to walk up the chutes leading to the killing room and lead others of his kind to their doom. There have been “Judas” steers, “Judas” sheep, and “Judas” hogs, which lived by betraying their kind.

 

FIGURATIVE USES

 

The Oxford English Dictionary (3^rd edition, 2013) has recorded an early figurative use of Judas sheep, from—I quote this dictionary—the “North America” (Philadelphia, Pennsylvania) of Thursday 26^th September 1907:

Professor Thompson acted as the ‘Judas’ sheep for the herd of dupes in New Jersey whom he led to the Storey Cotton Company’s shambles.

The earliest figurative uses of Judas sheep that I, for my part, have found designate the Democratic politician James Thomas Heflin (1869-1951), who served as U.S. Senator from Alabama from 1920 to 1931. During the 1928 presidential campaign, Heflin supported, instead of the Democratic candidate, Al Smith, the Republican candidate, Herbert Hoover, who was elected President of the United States; however, Heflin eventually abstained from voting for either Smith or Hoover. The following for example is from The Evening Sun (Baltimore, Maryland) of Friday 6^th December 1929:

Some time after the election Heflin blandly declared that he had sneaked out of voting for Presidential electors. This is said to have disillusioned and disappointed some of Heflin’s followers. They are calling him the “Judas sheep”—he that led them to the slaughter, but ducked before the ax fell on his neck, yet kept his ducking a profound secret until after the election.

Likewise, the following is from The Montgomery Advertiser (Montgomery, Alabama) of Thursday 14^th October 1937:

We do know that in Dothan in the fall of 1928 Tom Heflin, in the heat of a turbulent speech took an oath before the living God that in November he would vote against Al Smith for President, but that on the fateful election day in November he took witnesses to the polls in LaFayette to observe that he refrained from voting for either Hoover or Smith.
But he had already led thousands of his faithful Alabama followers to the slaughter.
They did not hesitate to vote for the fat Hoover and his high collar, because Our Tom had advised them to do so.
After the election many of them denounced their leader as a Judas Sheep.

The earliest figurative use of Judas goat that I have found is from the column Contract Bridge, by Ely Culbertson (Elie Almon Culbertson – 1891-1955), published in many U.S. newspapers on Tuesday 3^rd September 1935—for example in the St. Louis Globe-Democrat (St. Louis, Missouri):

My most sincere admiration goes to the player who is resourceful enough to put his opponents in a position to guess wrong—particularly if he can contrive some misleading hint in the fall of his own cards. Just as the expedition of the flock from railroad to slaughterhouse is led by a trained goat (called for obvious reasons the Judas goat) so too the clever declarer can sometimes provide a Judas goat to lead the opponents astray.