Thank you for the info @ Roxo. I'm sorry, but this happens to be exactly what I suspected you would say.
It appears UO is processing the Ethereal Mount as the mounted pet, creating a conflict with the previously mounted pet (Wildfire Ostard), whose mount/pet status was not properly set when a forcible dismount occurred. In order to remedy the two mount conflict, the game retains the current mount (Ethereal mount in your case) and removes the previous mount (Wildfire Ostard). If the mount removed is a tamed mount (not from a statue) the mount can still be found and reclaimed. Unfortunately, if the mount originates from a statue, the game deletes the mount.
This has been a known and reproducible bug since October 2019.
Paroxysmus Swamp Dragon just gone *poof* - Ultima Online Forums (uo.com)I personally think the disappearing bug is because the games "sees" the pet as derived from a statue and tries to return the pet to statue form. But only after the forcible dismount bug triggers the two pet conflict.
My suspicion is that when the first statue pets were introduced, the core of the ethereal mount programming was repurposed. Ethereal mounts return to statue form, but the code for other pets to return to statue form was never created (why would it be, that wasn't the intent). So, after the forcible dismount conflict occurs, the code runs to clear the conflicting pets, which the game sees as return to statue, and the pet disappears, but no statue is created.
I'm guessing the bug is extremely messy and tamed (not derived from a statue) pets are wiped from the follower list, just as if they had been returned to a statue, but no statue is created, and the pet is left standing around.
That is my gut instinct at least.
This doesn't help you with your pet loss at all and I'm sorry yet another player has lost a pet, but at least you have some type of explanation. If you follow the thread linked above you can see exactly how the bug works.