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1.
A teen boy who is hung like a horse (9 inches or more) and full of sexual stamina.
Oh my god, I caught Sarah sucking Eric's cock. That kid is a real Horse Boy..he was enormous!

Randy fucked me for most of the night with that giant cock, what a Horse Boy.
by Ripped Jeans August 10, 2010
 
2.
noun: A boy (or man) who loves horses (in a very intimate way)
Mark is a big horseboy
by ALS Project May 10, 2003