You must either Poke Transfer a Mewtwo from HeartGold/SoulSilver or there is currently a Wifi Event going on for all English language Black/White games to receive a special Mewtwo for free. This event will last until March 6th, 2012.
There was also a similar event occurring for Japanese Black/White carts back in 2011.
Mewthree doesn't exist.
Yes
if you have the action replay
Mewtwo learns Aura Sphere at level 93.
There is no mystery gift to get keldeo.Keldeo is the mystery gift but it come out when the new black and white 2 movie comes out only in Japan.
Zekrom (Pokemon White) and Reshiram (Pokemon Black).
There are no pokemon after pokemon blACK and white but there will be more pokemon games in the future.
it is not one of the ancients you can catch
No but you can transfer from Leafgreen,Firered to HeartGold,SoulSilver then trade to Black,White!
if you have it on HG, SS, Diamond, Pearl or Platinum yes
trade
Mewtwo is not in the game itself you will have to transfer it over.
If you somehow manage to get it on HeartGold, SoulSilver, Diamond, Pearl, or Platinum, you can trade it to Black or White using the thingy.
You can only get it in Pokemon chaos black version. Its is where mewtwo is instead of mewtwo.
if you have the action replay
I'm not sure but I think you have to complete the game and go to mystery girt and it will say Mewtwo returns click on it and there will be a guy that's wearing blue and he will give you a lv70 mewtwo im not 100% sure
mewtwo is not caught normally in black in white but you can get it off a trade or wi-fi. it may also appear in the dream world but im unsure.
the most powerfull Pokemon is mewtwo but people think its areceus when its not on Pokemon explores of time it is darkrai on darkness it is mewtwo on platinum it is mewtwo on dimond it is mewtwo on pearl it is mewtwo on blue rescue team it is mewtwo on red rescue team it is mewtwo on chores black it is mewtwo on ruby it is once again mewtwo i could go on for ever but i am trieng to make my point :)
You can't, but you can catch Mewtwo at the end if Cerulien (Don't know if i spelt this right) Cave.