Deoxys has numerous forms. And since Mewtwo and Deoxys are both artificially-made Pokémon (Deoxys from a virus, Mewtwo from cloning Mew, if memory serves correctly), it only makes sense that there exists the possibility to manipulate characteristics of Mew into Mewtwo, and the characteristics of Mewtwo into yet another form.
Besides, the new form looks awesome. Weird (what with the tail moving onto its head and whatnot--it sort of reminds me of Mewtwo mixed with the
Xenomorph from the movie Alien), but awesome.
“…Judge not what a man has done, but judge what he could have done if he was a different bloke altogether. For art thou a leper? And a leper can changeth his spots…” --Rudy Wade, Misfits (Series 4, Episode 8)