I'm not sure what you mean by elevated. If you mean whe […]
I'm not sure what you mean by elevated. If you mean when loading; being elevated above the ship, then that's not true. Horses were loaded onto ship by many different means, including slings and ramps.China Ship Elevator Suppliers
If you mean they had to be on deck (topside), then it was likely because prior to the 1900's, most belowdecks had poor ventilation, and the mortality rate for horses - critical and prized for transportation, warfare, and agriculture - was noted as being higher for those transported below. The cost of the horses was too precious; the loss of one was a significant price.
If you mean they had to be standing, it's because that is how horses sleep, eat, and live: upright, on their feet, as most prey animals do. Horses may lie down and roll, but they aren't happy being kept that way, and their most necessary muscle groups will atrophy.
If this doesn't answer your question, feel free to clarify, and I can try to share any other trivia....