A hot, humid day. One rider, one horse. Both are exercising at a moderate level, but who is more likely to overheat? Your horse gets hotter much faster.
Health
Can older horses be affected by senility or dementia, similar to elderly humans? Surprisingly, there has been very little medical research in this area.
Like people, horses can experience allergic reactions or side effects from a wide variety of substances or drug combinations, some quite serious.