Hipica Vilaller is the best mountain horse stable I've tried to date. The owner looks and acts like Christopher LLoyd. I went with some friends and he had us dying with laughter half the time, but was also an excellent instructor beg-adv.
He only speaks Spanish or Catalan, but managed to sing a rousing "Happy Birthday" to our English friend, and was able to instruct him pretty well even without our translating.
For 3 hrs he charged 40€ pp, and for 5 days/nts including horse, hotel, food, and picnics it's 710€
Address:
Vilaller,Alta Ribagorca, in the Pyranese Barcelona