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All extradimensional creatures are aliens, but not all aliens are extradimensional because if it's alien to this world in any way it can be considered an alien but just because said thing is an alien does not mean it is therefore extradimensional. That said there's no proof deoxys' virus didn't come from another dimension. Giratina is from another realm so id be fine with the theory of him as an ultrabeast. Same with the unown they are not specifically stated as being from any one place realm dimension etc.
