I know people really value seeing the bosses in From Software's games for the first time while they're playing, so I will throw up a spoiler warning for those guys. Don't tred forward if you want to be surprised by every boss in Bloodbourne. If you're not so worried, follow me below the warning.
The new boss, as revealed by https://www.youtube.com/watch?v=u0FKfK0H9dY, is the Darkbeast. It's a pretty gnarly looking skeleton, dogish thing with electricity running through its body. PlayStation Blog have given me a hand here by describing it as:
This electric beast emerges from giant skeletal remains. With lightning coursing through its body, it bombards the player with its erratic and off-speed movements
If you want to see it in action, take a look. This thing is really quite unsettling.