If the horse is truly underweight it wouldn't just hurt a horse (by putting lots of stress on it's heart and lungs, as well as legs and back), it would also hurt my self-respect to ride one. There is such a thing as compassion. You (general you) is NOT forced to the physical stress when you are malnourished and/or sick, and neither should be any live creature!
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."