Yum yum, this was bum...
Did not work for me. Now i have finished the hole story and still not able to buy. No horsie for sale in any shop
(just when the game concluded i briefly had the stallion to ride (bud not in inventory) but it disappeared when i desynced...
Seems like its the same bug as for outfits. If you have all horses (except Eternal Maw) you will have to sell a couple (regular) ones to be able to buy it. Just dont sell anything you cant buy later on