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Nihilego is based on the planet Mercury, the god Mercury/Hermes and the element mercury both in alchemy and as a real-world metal. Hermes was the dude with the winged sandals, he was the gods' messenger and symbolized the bridge between the mortal realm and the god's realm. This is like how Nihilego is the first UB you meet and sets off the plot with Ultra Space. Also besides the sandals, Hermes also wore a winged helmet, and Nihilego is literally a flying hat. Hermes' helmet was even typically a domed one with a brim like Nihilego.
For the metal, alchemic mercury is associated with femininity which is why Nihilego looks like a little girl. Mercury's elemental symbol is even just the standard "female symbol" with a little hat on it. As a real-world metal, mercury is highly toxic which is why Nihilego is Poison Type, and one of only 4 metals that are liquid at room temperature hence its jellyfish-ness (which are also poisonous).
They appear to be doing something rather clever here, having Nihilego based on the phrase "mad as a hatter". The origin mad hatter comes from the fact old hat companies used mercury in their apparel, and it didn't take long before there was a massive epidemic of mercury poisoning that literally drove people insane. That is what mercury poisoning does, mercury is most devastating to you neurologically (another reason for the jellyfish connection, jellyfish are just blobs of nervous system). Mercury poisoning tends to fuck up the brain, driving people either depressed or manic. That whole shit about Nihilego fusing with Lusamine and making her crazy is just a fantasy version of mercury poisoning. Nihilego is just the phrase "mad hatter" if the hat was literally an alien parasite.
All the original Ultra Beasts are also based on the planets, the gods they're named after, and the 7 principle metals in alchemy. And look I know I'm dropping a bombshell on you all here, but I am frankly VERY lazy and don't feel like elaborating at all lmao, sorry
