Hey,
when u clicked "dismiss" ?
Directly after clicking on the horse to buy it in the boutique? If yes, then u`ve not spent any crystals. In this case, buy it again.
If u`ve buy this mount and clicked right on the horse item, in ur boutique bag, and clicked then on "dismiss", then should the item still there. In this case click right again on this item.
If the item disappear and u haven`t the new mount in ur stable (u can find it if u press "j") then pls write a ticked.