Episode scene:

Judge Garth is buying horses from an old friend and brings along his top hands, Trampas and Steve. While the Judge is inside visiting, his friend’s son Hank (named after the Judge) is breaking a horse while the others watch admiringly. When the horse appears to be broken and is calmly standing under saddle, Hank suddenly pulls hard on the reins, hurting the horse's mouth and causing him to rear. Trampas’ look changes immediately from one of admiration to one of anger as he speaks the words above. Hank pulls a second time, then climbs down and smugly asks Trampas if he learned anything. Trampas replies sharply “How to ruin a good horse! The Judge would dock you a month’s pay for yanking on a horse’s mouth like that!” “Happens I ain’t hired out to the Judge,” Hank retorts.
Ironically, Hank’s father has just completed an agreement with Judge Garth which forces his unruly son to work at Shiloh where it’s hoped he will straighten out.

Quotation from the book (Chapter 26):

“Six o’clock already,” said Balaam, saddling the horses. “And we’ll not get started for ten minutes more.” Then he came to Pedro. “So you haven’t quit fooling yet, haven’t you?” he exclaimed, for the pony shrank as he lifted the bridle. “Take that for your sore mouth!” and he rammed the bit in, at which Pedro flung back and reared.
“Well, I never saw Pedro act that way yet,” said the Virginian....“Mr. Balaam...I’ll buy that hawss off yu’ right now.”
Balaam shook his head. “You’ll not do that right now or any other time,” said he. “I happen to want him.” The Virginian could do no more.
Meanwhile Balaam began to lead Pedro to the creek for a last drink before starting across the torrid drought. The horse held back on the rein a little, and Balaam turned and cut the whip across his forehead. A delay of forcing and backing followed, while the Virginian, already in the saddle, waited...
“He ain’ goin’ to follow you while you’re beatin’ his haid,” the Southerner at length remarked.
“Do you think you can teach me anything about horses?” retorted Balaam.
“Well, it don’t look like I could,” said the Virginian, lazily.
“Then don’t try it, so long as it’s not your horse, my friend.”
Again the Southerner levelled his eye on Balaam. “All right,” he said, in the same gentle voice. “And don’t you call me your friend. You’ve made that mistake twiced.”

Additional comments:

1. The tension between Hank and Trampas escalates throughout Hank’s stay at Shiloh until it ends in a showdown between the two. Though Trampas does all he can to avoid it, he is forced to kill Hank. This is similar to the Virginian’s reluctance to force the issue with Balaam regarding his treatment of the horse Pedro. Eventually, after Balaam inflicts an unspeakable injury to Pedro (gouging out his eye!), the Virginian lets loose his anger and almost beats Balaam to death.