The horse meat that I buy is predominatingly very lean.
But sometimese it has got a little bit of fat; like that one.
I don't know much about horse meat as I have never tried it or saw it for sale here. Only from what I know I saying that the grain-fed steaks I buy have similar marbling sometimes (marbling is the term when the fat is woven throughout the muscle meat)...and the grass-fed steaks I buy have no marbling at all. Deer and goat I get has no marbling either. Lamb I get has plenty of fat on the sides and around the bone but non throughout the muscle meat parts.
When it comes to the sheep this theory is bullocks 
The mutton is SO FATTY at the end of the summer, from animals eating ONLY grass, wild fruits, etc.
So much internal fat.
Yes a lot of fat but its all connected to the meat as a pose to being evenly marbled throughout, thats the main difference I see.
The taste of badly fed animals meat can be great ....
Any vindication? I disagree.
Tyler probably too.
Yep, "badly fed" meaning not starved or anything, but giving animals plenty of grain will cause marbling. They can taste great if you get some of the nice expensive marbled steaks they taste divine. There is a lot of grain-fed meat that can taste like crap yes, but the "good" marbled meat is really good, even if you like the grass-fed stuff you can't say its not tasty, unless you haven't had a truly good marbled cow steak.
Again I have no idea because its possible some meats can marble on grass only!
However I doubt any wild ruminants will develop marbling, ever.
Heres a diagram of marbliness I found:
