I am puzzled at your comparison of deer and horse. I’ve helped butcher deer. There is barely any fat inside the muscle, just a thin layer around some cuts. The deer on my property may sleep during some of the day, but we see them at all hours and they run, walk and move most of the time. Compared to grazing horses I’ve observed, the deer can be more highly active. The local deer feed on corn and berries.
The key to tender deer meat is hanging, short periods of marinade and quick cooking. The large muscle cuts are tender and tasty. The tougher cuts are ground. The meat in the neck and legs is full of tendon and can even be difficult to grind, as the tendon will wrap around the auger and clog the plate. I simmer these cuts until tender and use them for dog food. The collagen in the connective tissue is good for the dog.
I’ve spoken with people who have eaten horse. One comment heard a lot is that it is *sweeter* than beef.
I’d try it and make up my own mind.
Deer get a lot more activity than horses. Deer have to search for food, water and shelter, often times over a territory of a square mile or more depending on where their food is in comparison to their water and shelter.
Humans on the other hand confine horses in fences and bring their food and water to them.
Although I am a horse lover, I would like to try horse meat sometime.