In Game of Thrones s01e05 "The Wolf and the Lion", Ser Gregor Clegane (The Mountain) slaughtered his horse just after being defeated by Ser Loras in a jousting competition.
My question is why did The Mountain do that?
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Little finger explains it right after it happens, with Sansa saying that Loras would never resort to cheating like that.
Littlefinger: "Loras knew his mare was in heat. Quite crafty, really."
Sansa Stark: "Ser Loras would never do that. There is no honor in tricks."
Littlefinger: "No honor, but quite a bit of gold."
Notice that the Mountain's horse:
The second tilt of the day is between the fearsome Ser Gregor Clegane, the Mountain, and the popular 'Knight of the Flowers', Ser Loras Tyrell. Loras gives a beautiful flower to Sansa Stark, while Littlefinger and Lord Renly make a wager on who will win with Renly betting on Loras. Loras is riding a mare that is in-heat, which confuses the Mountain's horse, causing him to throw his rider. Enraged at having lost, Gregor first decapitates his horse and then attacks Ser Loras.