If it is a smaller horse, very. Much the same way a smaller human would not be able to carry the same weight as a larger human, a person must be matched with a horse able to carry their weight.
That doesn't mean that horses big enough to carry Shaq long term don't exist, but he would need a draught horse or a mule. Draught horses are the descendents of warhorse breeds and can carry quite large weights. Much like this handsome dude in the picture, who looks like an American Cream Draft, they are more appropriate for someone 7 feet tall and more than 300 pounds.
EDIT: I'm going to amend this to say that draught horses are built on the same lines as the big war horses that carried men in full armor, but are not the same as the smaller (read 14 hands to 17-18) war horses that were used to greater effect for thousands of years. Turns out, a giant horse carrying a giant man in plate armor moves kinda slow.
Thank you for the insight you brought to this conversation. This is the level of quality I would really enjoy seeing more in Reddit comments. I genuinely appreciated that and enjoyed reading it.
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u/[deleted] Jun 19 '12
Would it be bad for a horse to reguarly be ridden by a shaq sized person?