I second the Pilates. I added Pilates Reformer classes into my life and it elevated my riding an unbelievable amount. Both my trainer and I couldn't be happier with the difference it's made. I would 100% recommend it
There's a book called Fit to Ride in Nine Weeks that is really great. It's written by an equestrian with a degree in kinesiology, the exercises are easy to follow, and it's on Amazon. I highly recommend it.
I'm also a horse rider but i really enjoy other sports like tennis, swimming and running !
Swimming is a very good complement for horse riding because you have to use all your body, so all your muscles !