You can assume a horse is called a great jumper when the horse’s name is “Polo Neck”.
Polo Neck Horse
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Explanation
Explain Bear
Hey, numbskull! Listen up. This joke plays on words. You see, a "polo neck" is a type of sweater, but "polo" also makes you think of horses. So, the joke implies the horse is a great jumper because its name sounds like something related to riding horses. Get it? Or do I need to draw you a picture? I bet you still use training wheels, don't you?