I think using the admin panel and teleporting your character to your Thrall might be the best solution, but in case you don’t know how to do that or cannot access admin panel on official server, here’s what I found:
Today my horse fell under the ground while guarding, and it initially seemed impossible to get it out. I removed my bracelet, I relogged, I tried waiting for several minutes for it re-appear, but none of that fixed the issue.
A few meters ahead some crocodiles kept trying to attack me when in range. I noticed while fighting one that I was able to see the health bar of my horse somewhere nearby at the ground, but the horse itself was nowhere to be seen. I knew it was my horse because of the rapid movements it makes in panic when enemies are near. This is how I realised it had somehow stuck underground.
I was unable to find any solution in the forums and most comments basically suggested quitting. However, because I was still able to see my horse’s HP, I grabbed some health potions and decided to pull a beast near the area where my Horse was supposed to be guarding and allow it to chase me around in that area. I was able to see my horse’s HP as it run in different directions trying to ‘‘escape the beast’’ all the time while under the ground. I persisted for a few minutes, and in the meantime I destroyed all nearby foundations and walls to make it easier for the horse to get out. And won’t you believe it, it actually came out of the ground! I know this is not an outright solution but it’s a tip for those who really want their Thrall/Horse back as well as the items they had stored in it.
To sum it up: Grab some health potions and bring an enemy near the exact spot where your Thrall/Horse is. Allow the enemy to chase you around that spot. Punching the enemy once or twice will reveal the HP bar of both the enemy and your Thrall/Horse as separate health bars. Follow the HP bar of your Thrall/Horse with the enemy chasing you. Keep it up until you manage to get your Thrall/Horse to emerge from the ground/terrain.