Again, I had to keep following the horses around. I took many other shots from many other angles. I also did several different exposures for each shot. And did shots with and without a polarizer.
For other shots, like this one, I did use a shallower depth of field, with a longer focal length. The one in the OP was at the wide end of my 24-120 lens, and on a film camera; it’s hard to isolate the background at that length because the depth of field is very deep even at the large apertures.
I LOVE that picture. He looks like he’s on his way to his stall on 7am Sunday after 24 hours of sowing some pretty wild oats.
Yes, it had rained a few days prior, and the horses were muddy. I thought it added character.
I try to be understanding and supportive of my friends’ art, but “hung like TWO horses” is not necessarily a compliment. 
Yep - what a muddy pretty horse! He looks happy. Good picture - and much better than the first one!
These horses are probably trained and rideable and should respond to some voice commands from someone on the ground. You could ask the owner to be sure - if you ever do this again, you might be able to get the horses to stop approaching you with a well-timed halt or whoa command. Though they are horses - they might know what you want them to do, but choose to not do it anyways!