Story on personification of a horse?

Question

Part of my story is that a horse basically becomes useless to humans. How/where would a stallion be injured so that it can still walk, but can't run anymore?

Answer 1

It could founder (laminitis) - It could develope Navicular.  (a severe pain down around the ankle from a tendon rubbing a bone,)

I assume you mean a racing run, not just a lope across the field.