This was another design choice intended to push the play of two-color decks
As Polygon notes, The Pokmon Company has made clear that, when fans distribute art featuring any of the companys copyrighted intellectual property, they are agreeing to a royalty-free, non-exclusive, irrevocable, transferable, sub-licensable, worldwide license that gives it the right to use, transmit, copy, modify, and display Fan Art (and its derivatives) for any purpose. Legally speaking, that means TPC would have been in the clear to keep the immersive Ho-Oh as is
That is, unless you play her on top of another Ariel you already have in play
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