This is less "Blow your mind" but more along the lines of clearing up a common misconception. I searched and this was mentioned in the thread earlier but only very briefly about five years ago so thought a more detailed post might be helpful.
Bowser/Koopa was actually NOT inspired by the Kappa creatures from Japanese folklore. Bowser was originally a monstrous Ox-like creature that Miyamoto partly based off the Ox king from Toei's Alakazam the Great:
You can see a version of Bowser's original blue "ox" form (without horns) on the original Famicom cover for SMB (along with weird, rag doll shaped Peach):
While going over character planning for SMB, Mr. Tezuka pointed out to Miyamoto that Bowser actually looked more like a turtle, and since most of the other enemies in the game were based on turtles, they decided to change Bowser's appearance to be more turtle-like. Miyamoto remarked that he could make Bowser "look cool now." If it hadn't been for Tezuka, Bowser and the Koopalings might have all ended up looking like odd Ox-like creatures instead of more like turtles/dragons.
How about the name Koopa/Kuppa? It would seem obvious that it's a variation of Kappa, right? Wrong. Like many beloved Japanese characters, Bowser is actually named after food. Koopa in Japan is "Kuppa" which isn't taken from "Kappa," Kuppa is actually the Japanese word for gukbap, a popular Korean rice soup. Other names Miyamoto originally considered were Yukke and Binbinba, which like kuppa/gukbap are also Korean dishes (yukehoe & bibimbap). Although the lake/cliff on the way to the first Switch palace in the original Japanese version of SMW is called "Kappa mountain," this is apparently just an unrelated visual gag that was put into the game for random fun.
In Japanese folklore, kappa often kidnap women/damsels. However, kidnapping women/princesses/other key characters is probably just a simple carry over from Donkey Kong and
perhaps also the running conflict of Bluto constantly kidnapping Olive Oyl which
theoretically could have been a possibility for inspiration as well (since Miyamoto loves Popeye and wanted to make a game with its characters originally before creating Donkey Kong after King Features denied the rights to using their characters at that time).
So although Bowser and the Koopalings seem to vaguely resemble kappa and seem to act like them, it's purely coincidental and they simply are what they are: simple, mischievous, yet funny and lovable villains from the ingenious minds of Miyamoto and Tezuka.
Taken from various sources/quotes and links compiled in the wiki for Bowser:
http://en.wikipedia.org/wiki/Bowser_(character)