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Ho-oh is the rainbow Pokemon, but is there any rendition of it that either shows it with rainbow feathers in a way that doesn't look like shit, or otherwise using rainbows in a cool way?
Rainbows are notoriously difficult to make look decent in designs. I've heard it theorized that Ho-oh was supposed to incorporate rainbows in its design (after all, Gameboy Color), but due to the system limitations it wasn't really possible.