Kill him and rez him. :P It'll go away. Just go pull a bunch of mobs somewhere. You could also put in a ticket to have a GM do it, but really.
It comes from sticking a pet in the stables with temporary buffs applied, if I remember right. It is an exploit to use/keep it otherwise, since you can get things like 25 day bestial wraths. D:
But I didn't even have him stabled beforehand, I was using him to run Sethekk Halls (curse you, Raven Lord!).
I don't see why he would've kept the buff like that though, there was a very long pause between me leaving the dungeon and getting to Orgrimmar.
._o
I'd probably report it as a bug, and then make sure you didn't use that pet for anything inappropriate (like pvp... or dungeons, etc) until it was fixed by either you, or a GM. Report especially if it doesn't seem to be one of the regular ways the bug happens, as Blizz might not know about it to even TRY to fix it, if they wanted to.