I stand behind what I posted and also what Spurious posted. Your analysis between humans and horses is very interesting but I'm afraid that I know nothing about equine physiology to comment, although what you did post motivates me to learn more about this. Nice post. Again, humans evolved to walk upright as they emerged into the open savannah to maximize cooling efficiency - basic geometry. At noon, an upright horizontally exposed quadraped has 100% of its topside exposed to the sun's rays. If the same quadraped were to be vertical, only about 5 or 10% of the previously exposed upright surface would be subject to the sun's rays. The rest of the body surface is then much more efficiently cooled by horizontal air flow - wind. Tremendous survival advantage.