The game removes arena creatures without Tribe Log lines, so report a missing creature the same day. An admin can read the hourly saves for its position and stats, move a creature that is still alive back to you (ASE), or consider a restore from backup; any replacement is the owner's decision.
Tell an admin the map, the time (the clock in your profile menu shows the server's time), your character's name, what you carried or had uploaded, and the exact message on the screen. For a transfer: both maps and both times. Report the same day — the backups reach back only about two days. Contact admin is in your profile menu; the Guides page has a question box.