Hello Ownedcore!
Batte Horn breaks rogues and druids stealth when used as posted earlier here (Battle Horn in Arena).
Now, this item is a little bit hard to obtain since it drops off a rare in pandaria but Blizzard added this item to the game in patch 5.1 which is a LOT easier to obtain! It costs 90 darkmoon faire tickets.
Using this item will also break enemies stealth within 30 YARDS as the Battle Horn. It works in RBG's and normal BG's. I have yet not tried it in arena but my guess is that it works there aswell.
Confirmation that it works on wowhead.com:
Sources: [US+EU] Darkmoon Whistle Fun