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The Japanese frequently steal Western concepts, almost verbatim, when creating. Countless manga have scenes, names, or even entire premises taken directly out of American movies (usually 80’s). Lots of gen 1 Pokemon are based on pop culture concepts, like hypnotism, Uri Geller, genetic experiments of the time, etc, and Tangela seems very likely to me to be based on the Fry Kids from McDonalds. By “based on”, I don’t mean they wanted to make a Fry Kids mon, I mean the designer may have seen a piece of promotional material with one on it, and the “look” stuck in his head. Artists frequently come up with an original concept, only to realize later that it’s heavily inspired by something they enjoy or saw recently. It’s just the way people learn.
So I think that without a doubt, the design idea came from the Fry Kids, which is why it feels a bit out of place. Designs can be made in a process, or they can be “inspired”, and in this instance, it was “inspired”.