I’d say if you can handle training yourself to get to that level where you could safely (relatively speaking) take a stallion, then I don’t see why not
Of course it would take several months of training, I’d say 6-9 provided you have all of the proper toys to not only stretch you, but to do some deep anal play. Starting with smaller toys and working your way up until you have about the desired diameter; then using longer but soft toys to work around the bends, using those for a while until you feel that you can safely move up to something a little more firm, and finally a fully firm toy. The idea being that you straighten those curves out slowly over time as to not cause pain or damage. I’d say once you can take about 18-22 inches full force into yourself then you should be ready. Someone earlier mentioned elbow-deep fisting which I think is another great idea but after all of that time training I’d say you should be able to have sex with the horse (at least anally) on his terms versus just putting the tip in and holding his penis. I only feel that it’s the right thing to do to give not only yourself but the horse the maximum pleasure possible