Was logged in on both my accounts (one regular, one remix) when the achievement for the Reins of the Astral Emperor’s Serpent popped. The item was delivered on both accounts. I learned it on my remix account, then tried to delete it on retail, and it said the item could not be destroyed. As it’s already learned in the mount journal, there is no way to remove this item from inventory now.
Thanks.
I was getting ready to make my own post but found your complaint and I think mine is related. I found a Reins of the August Phoenix item on a character on my second account and I also can’t delete it. You can’t just drag it out of your bag to delete it as you get the error “This item cannot be destroyed.” You can’t sell it to a vendor. I’d like to get my bag slot back.
Not sure if related, but I just did Gear Update on the character where this occurred. I don’t even know if it was initially logged on when I was rewarded the mount in Remix.
Yeah, seems like they’re not going to bother fixing this 
I had to get a GM to delete it for me. It took a few tries before one was motivated enough to help. Most just copy/paste a blurb about making more bug reports, which ends up accomplishing nothing. But at least I finally got that precious bag slot back!