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Nothing is based on nothing. Even if you're talking consciously basing something on legends and animals and things, there's nothing truly original, you're always taking inspiration from somewhere.
Not that that's a bad thing, it's how you combine and hone the elements of something to make a Pokemon that makes it well-designed or not.
A monster with no inspiration from the real world would be horribly Lovecraftian - something we couldn't even comprehend.