Mew, I'd say, only created "organic" Pokemon. Creation Legends and Pixies are a given, but I'd say some, Bannette, Deoxys, Claydol(?), Porygon, and what are now Raikou, Entei, and Suicune, to name a few, Could be separate from average Pokemon. They obviously abide by the rules (read: game mechanics) of other Pokemon, but it could simply be that the "soul" of a pokemon is in it, and it's DNA and defining traits common to all Pokemon shift some to fit that. Or it was 1stGen and the devs hadn't defined Pokemon as such.
More on-topic, Mewtwo was supposed to be Mew, human, Alakazam, and I believe Machoke or some such. I'm not sure where I remember this from, but it's somewhere.
If they tried cloning from Mewtwo they'd either make a glorious beast with a God-am-I problem, or an Eldritch Abomination of some kind. =/ I can't imagine them continuing that in the foreseeable future, however.