I had this happen today. My elephant always stays on my galleon. I checked my company logs and found the same entry about auto-decay and someone else claiming. There is a feeding trough there and it still had food in it this morning. I was last on the ship with the elephant yesterday. The bears and rhino on the ship are still there. The funny thing is someone left a bear on the beach and its still there with 6ish days remaining to claim. Seems like its probably a bug that has to do with large animals falling through ships as that has also been something I've been dealing with recently.
Edit: This also got me thinking throughout the day. The tames falling through ships seems to be a client side thing. If I log off and spawn in an area about one galleon length away from the ship I can see the ship rendering in and the animals render first and fall through while the boat's floors are rendering in. Could it be that somehow on the client side of the people who tamed our animals that the animals were rendered or forced to render further away from the ships and over a period of time became claimable? Is the decay timer server side? I sure hope so. However is the tame location client side? Seems so if the tame can fall through a ship based on where I log on. If I log off and back on while on the ship, I don't have the problem with them falling through. Also I don't seem to have that issue if I relog far away enough that the ship isn't even rendered.