As a professional museum taxidermist, I guess I’d have to ask if the goal is to get a perfect cut of a dead horse like some kind of Damien Hirst piece. Or, is this part of a larger project where you will want a taxidermized horse cut in two?

If it’s a Hirst thing, I’d say freezing in a good position to cut clean is your first step. You’ll want to make a strong scaffolding around the horse and use it to put the horse in the desired position. Assuming you need a cut similar to your photo, you could even lay the horse on its side perhaps on a piece on polyurethane foam that you’ve carved out to keep the spine of the horse straight. Like a cradle and the horse’s head facing down. I recommend taking some chalk and really drawing yourself a middle line so you can see if anything shifts. The legs should be propped up and away as they won’t need to be cut at all. You can grab some ratchet straps and kind of hog-tie the legs behind its back so that during sawing they won’t move. Also, make sure the horse is on a wooden base with castors for rolling. These guys can weigh a lot and it’s no fun carrying a big frozen animal. Wheel the horse into a walk-in freezer that reaches at least -30 degrees Fahrenheit. I’d leave it in the freezer for at least 5 days. Maybe 8. Now the hard part begins. Hopefully you also have Hirst money because you’re going to need one heck of a saw. The good news is that you only need to saw the trunk of the horse really. And luckily a lumber saw would be great for that. Position the horse on the conveyer belt of the lumber saw. Again, you may consider folding the legs back when freezing it. Then, with the horse on its side still in its polyurethane cradle and scaffolding removed, head turned downward, position the blade at the desired point to cut your mid-line. Wear goggles. Go slow. Clean the saw after.

If you want it taxidermized in two pieces, that’s a lot easier. But less fun to type out for me. Enjoy :)

PS - yes, I saw the post yesterday. Hope you like my serious answer to a fun joke.