Here he is, the big bad himself. Mr. 150.

Mewtwo is the closest thing the Pokemon games have ever had to a final boss, and does he deliver?
Hell to the yes.
Mewtwo is a psychotic psychic with the voice of Dan Green and the temperament of an omnicidal maniac. When he was created to be the world's strongest Pokemon, everything went horribly right.
Mewtwo is a badass son of a bitch who can turn your brain into spaghetti with a glare, and conjure up world-drowning storms with a wave of his hand. He wears badass armor at one point, but then says "screw it" and wreck's Giovanni's shit before doing his own thing.
Which is either creating an army of clones to rule a perfect world, or sitting around in a cave, depending one whether you're watching the anime or playing the game.
Special mention must be given to manga Mewtwo, who wields a battle spoon and is bound by his DNA to Blaine, the Cinnnibar Gym Leader.

Yes, this really happens.
anyway, just what sort of animal is Mewtwo supposed to be? I've heard cat, but he doesn't really look it at all. Not his ears, but his head kinda looks a little feline. Also, what's up with that second neck? That always looked really distinctive to me.
I think that's part of why Mewtwo is so threatening: he looks like he might have been a recognizable creature at some point, but he's been twisted and warped into something else. He totally looks like a god-tier lab rat, and finally capturing him is the feater in everyone's Pokemon cap.
Overall: 10/10