It's in the snapshot 12w34a and 12w34b. It's a new "Boss Mob" with 300 hp. It has two fist that look like its head, that shoot canibals like its face. It attacks any passive mob, and when it attacks you get a wither effect which turns you health black and you loose health over time. The only way to get to it is through MCEdit filters, but theirs some videos on it. Theres allot of cool new features in the snapshots too.