Endurance seemed like the best forum in which to post this.
After the RMH Hunter Pace on Sunday, barely feeling anything yesterday but feeling a bit stiff today first thing, I began wondering why it is only the 2nd day after an unaccustomed long ride that makes one feel the crunchiest.
Discovered this is a very well-defined phenomenon:
Yup, I was diagnosed with that years ago. DOMS is when you feel fine during the activity but 24-48 hours later everything seizes up and the pain REALLY sets in. With DOMS you end up doing a LOT more than you should because while engaged in the activity you feel strong, and fine.
It's good to remember for your horse also because despite the pain, you both should continue to exercise moderately during this 24-72 hour period to lesson the length and intensity of the muscle soreness.