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If I remember correctly, the imperial warhorse was one of the rewards from the mysterious reward boxes that were given out during the 10 million players event.
Most of the time you could count on items like this returning in crown crates to make ZOS more cash, but in a strangely out of character play they decided to lock the imperial warhorse, dark shaman, treasure hunter, chef, blacksmith, and alliance war dog behind a week long in-game event that was dubbed a celebration for reaching the 10 million total players milestone.
Unfortunately, it quickly became apparent that this was no "celebration", but a thinly veiled attempt to stress test the dungeon finder. In order to earn mysterious reward boxes players were required to log in and complete a random dungeon through the dungeon finder each day, with the ability to earn one box per character.
As one might expect, the dungeon finder was not up to the task and fell apart repeatedly throughout the week despite attempts made by ZOS to fix it, thus resulting in many players not being able to earn many, if any, reward boxes. To add insult to injury, the drop rates for the collectibles were absurdly low. Instead the reward box loot tables were heavily weighted to give out crown potions and treasure maps. ZOS even saw fit to include single lock picks and basic ingredients in the loot table. Whether this was meant as a slight to the players or just a joke made in very poor taste is unclear.
It is likely that ZOS has no intention of adding the imperial warhorse or other event collectibles to the store any time soon. The good news is that ZOS Gina Bruno recently made a statement that there would be another chance to earn mysterious reward boxes, and that details would be shared when the date of the event is a little closer.
For now we can only speculate as to when the event might be, and whether or not the manner of obtaining the boxes, and the loot tables for the contents, will be the same or not. I sincerely hope ZOS rethinks the manner in which they conduct these events because the last one, described as a celebration of ZOS loyal fans, left many feeling anything but appreciated.