Ever heard of ECVM?
But I watched a fascinating presentation on it.
ECVM is Equine Complex Verterbra Malformation. It's a malformation of C6 and C7 of the horse's cervical spine that causes asymmetries in the ribs, muscles, and can affect some major nerves.
It appears to be genetic/hereditary. And it's showing up in many breeds because it seems to be very prevalent in Thoroughbreds.
If you are interested in equine biomechanics or physiology I would totally recommend watching this video. It's about two hours long but I was enthralled the whole time.
Also - good luck not looking for symptoms in a horse you know (or have known)!