Perhaps you could set up a "studio" in a vacant stall in the barn? (Don't know the arrangements there for your horse, so I'm guessing. Perhaps, if the current space does not exist, you could add onto the existing stucture to accomodate a space dedicated, and large enough, to allow "controlled-light" portaits of horses?

Just thinking out loud.

...and if you're feeding the bills TO the horse each month, just tell the collectors "my horse ate it" when they come knocking - see, no need to actually PAY the bills, just let the horse deal with it all... LOL