Nyxathid-Stiflable?

[jazzykat]

Is the choose opponent part of Nyxathid stiflable? What happens if it is? Does it stay a 7/7 no matter what?

[Skeggi]

No it's not. 'As (...) enters the battlefield' is a replacement effect, not a triggered one and therefor not 'stiffable'.

However, if you would copy a Nyxathid with some sort of shapeshifter already in play, like Vesuvan Doppelganger, you won't get to chose an opponent and the second ability (getting -1/-1) won't do anything. So in that scenario, the Vesuvan Doppelganger would be 7/7.