The Hirukon rare spawn in Zereth Mortis that drops the Deepstar Aurelid mount is currently tradeable under certain conditions.
Once you have the lure that is used to summon Hirukon from the water which guarantees the drop as long as you don't have it (this is important) you can trade it once per day to anyone that killed it with you in your party.
You can only kill rares once per day as per usual. To summon him every day, you must log out after you have summoned, killed him and traded the mount to your friend/guildie to maintain the 15 minute buff you get when you consume the lure that Lady Vashj gives you once per week.
Trading the mount also works cross realm as long as the player is in your group when you kill Hirukon. I haven't tested it cross faction.
If you have the lure and have already learned the mount, this will not work for you.