I have been on the carnivore diet for over 9 years and will not be altering my way of eating anytime soon. I am not a horse person, so I can't speak specifically to that.
I had a proctocolectomy in 2015, so I have a permanent ileostomy. I wish I had known about the carnivore diet before UC finished destroying my colon and I had to have it removed. I still ate a paleo diet for a year after the surgery because I didn't find carnivore until 2016.
Fiber is your enemy (despite what we have all been told forever), and you can get everything you need from meat, especially ruminant meat.
You will definitely be able to get back to barrel racing when you and your doctor feel you're ready. I started back to playing full court basketball 3 days a week about 4 months (I don't remember exactly) after surgery, but I maybe could have started back sooner. I was eating plenty of meat, but also a pretty good amount of vegetables, as the paleo diet recommends. So my digestion was still gassy and painful a lot of the time.
When I dropped all plants from my diet, my digestion became smooth and quiet almost immediately, and continues to be.
I would suggest NOT consulting a registered dietician because they will tell you to go plant-based and eat a bunch of leafy greens and such. That is the absolute worst thing you could do for your gut, especially when you have just had major surgery.
I didn't really have a choice on the surgery—my colon had basically stopped functioning, but I'm glad it could be done and I could go back to living a "normal" life. The silver lining is that I'll never have to have another colonoscopy and colorectal cancer is one less cancer I have to worry about getting, ha!
I don't know if you do Facebook, but there is a great group on there called Ostomy Lifestyle Athletes that you could get some great advice and inspiration from.
Please keep us updated!