Not sure if this has been discussed yet, but I recently verified the Hunter's hidden artifact* effect. Obvious spoilers as these are meant to be "hidden". For those of you who have no idea what the hell I'm talking about by hidden effects, see these threads:
http://www.mmo-champion.com/threads/...en-Cheat-Death
http://us.battle.net/wow/en/forum/to...16?page=11#202
The Hunter effect summons a not-so-massive Eagle which defends the player when they reach 40% health, on a 3m cooldown. It will attack your target for minor damage, stun the target for 4s and last's about 15s before fading away. It also looks like it's capable of taking aggro, but it doesn't taunt, and the low damage means that it probably won't take any aggro off of the player. A small note of minor interest is that the 3m CD clears on death, unlike other class effects.
https://www.warcraftlogs.com/reports.../#type=summary
*Note: I'm calling them artifact effects because they are tied to equipping an artifact, however it looks like the effects are class-wide rather than spec-specific, so it's the same effect regardless of which actual artifact you're using.