Offical name of the "boss" is the Elder Guardian, the small ones are Guardians. Also, Sponge available in survival! :D
I just want to say...I'm terrified of them. I've always been a scaredy-cat, and when the freaking face of the elder guardian went down through the screen...erg. Needless to say, I'll be staying the hell away from those things.
the potion effect has been around for ages, just never really used for anything xD, mining fatigue is one of those potion effects that normally couldn't be found as a potion or from a mob or beacon, as spike had said, it isnt old, it was just one of those hidden in the background for some time, it was and still is obtainable with command blocks, plugins and Too Many Items (Probably more)