I had Ergrott, Cerulean Terror (\aITEM 512022544 -1410345652 0 0 0:[Ergrott, Cerulean Terror]\/a) on one of my toons.I had all the unlockable barding slots unlocked, and had it trained to level 14. The mount is HEIRLOOM, and I decided I wanted to move it to a different toon, so I popped it into the shared bank and picked it up with the new toon. However, every bit of training and all the unlocked barding slots are GONE. Especially since you can sink US$ into training and Cobalt Crates for the barding slot unlockers (and yep, I used Cobalt Crate barding unlocks!!), once you have done that training and unlocking, it should stick, even if you do as I did and move the mount between characters on the same account. If I let my sister ride my Appaloosa here in BFE where I live, he doesn't suddenly forget all his training. Neither should Norrathian mounts.
Out of curiosity, move it back to the main character and see if the unlocked slots magically come back...
This happened to me, with the same mount, a few weeks ago. I took it off the character it was on and passed it down to another who was about to run the TS sig line for Hand of the Maker. Lost all training, did not have any barding unlocked so I don't know about that. I figured it was tied to the character and have not looked to see if it reverted back but I'm going to assume it won't. I'll check at some point. I can't be bothered to log in at this point.
Wow I thought this was a known feature or I would have posted it a month ago! The mount resets anew on any other toon you move it to. It DOES go back to whatever level of progression you've attained while on THAT character. For example, I maxed it out on my main...decided I'd start swapping it to other alts as I worked on them. Totally freaked out when to my utter disbelief my max level mount was back to level 1 again. I flipped it back to main and it was maxed, with all of the gear again. So, long story short...I have maxed it out on 3 characters now. BRAVO DarkPaw financial analysts that thought this up. Brilliant cash sink. If it's a bug, do the right thing and credit me the extra money I invested to level it. If it's working as intended, I applaud you with bitter vehemence.
I'm not really going to swap my now (again) fully-trained mount to an alt to check. It should be easy enough to reproduce. Give a toon a mount, train it to max. Then unequip the mount and pass it to another toon on that account, and summon the mount. The training goes away.
This is not a bug. Mount training is per character. If someone transfers it or gives it away it does not retain levels for the new character.