Horseback riding brought me to yoga, and the two seem to go well together - I ride better, and get more out of the activity and the horse I’m riding when I’ve been to the mat, and when I’ve done some meditation. He enjoys that more too!
What I struggle with is that the first ten - twenty minutes of my ride are painful as the muscles across the fronts of my thighs are painful after mounting. Everything I read suggests it’s the adductors that need to stretch to sit a horse, but my pain goes across the front of my thighs, and is worse the more I sit tall , or try to tuck my pelvis, and sit more on my “back pockets” - it feels like a hip flexor, and the tightness goes all the way to my knee, but I thought hip flexors would be contracted, not stretched when sitting a horse. There seems no posture or asana I can find in my limited understanding that prepares me or allows me to stretch for this. I am hoping someone else has dealt with this problem before. 
P.S I was the kid who always sat reverse tailor, rather than cross legged, and walked on the outside of my feet, with the toughest muscles from years of biking, on the outside of my calves.