Mewtwo does not come from any egg, so it is possible that it's a glitch egg, that if it hatchs, it ruins your game.
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Steps to Hatch: 4096
It takes 350 steps to hatch
It takes 5,120 steps to hatch Gligar in a Egg.
in my experience it takes about 5,355 steps to hatch a sneasel.
it takes about 10,240 steps to hatch a beldum egg