So I found myself wondering: why would someone fuck a horse; why would you think to make a porno out of it; how would you go about it?

You get yourself a couple of skanky porn queens, because everyone knows regular non-skanky porn queens wouldn't fuck a horse - they do have standards after all. You drive around in the country for a while until you see the perfect porn-star horse. Go up with skanky porn queens, camera crew, etc. and knock on the farmers door:

Porn Producer: "Uh, hey, can we, uh...borrow your horse for, uh, an hour or so?"

Farmer Bill: "What?"

PP: "Yeah, just, uh, need to, uh, get some footage for our, um...documentary."

FB: "What the fuck...?!?!?!"

PP: "We're with the, um, uh...Discovery Channel"

FB: "Well, I reckon..."

PP: "Good, um, it would be great if you didn't, uh, watch, for, um...authenticity"

FB: "..."

PP: "Say, that's a 'he' horse, isn't it?"

FB: "Yeah..."

PP: "Good..."

FB: "You're not gonna hurt him now..."

PP: "No, I imagine he'll quite enjoy it."

FB: "Wha.."

PP: "Here's an old gym bag full of money."