Good news and bad news. Good news, the new mob is in newest snapshot. It's fairly hard and has an even harder boss-like variant referred to as an "Elder." The mob's name is officially "Guardian." It attacks by firing lazers from it's eye. It has a secondary attack to, but not a direct melee type. See in wiki entry.
http://minecraft.gamepedia.com/Guardian
Bad news is Bunnies aren't, but as consolation, this is snapshot 14w25a, not the 1.8 pre-release, so they may still make it in for 1.8. Also, underwater temples/ ocean temples do seem awesome and have a new block. Oh, and you get can sponges legitimately, since the Elder variant drops them and they seem like they're meant respawn, just like the normal Guardians.