First off, Eternatus's height. I've seen via the Japanese Pokemon site, they seem to use metric, which is probably true in the Japanese games, too.
Cosmoem is 999.9 kilograms heavy. And Eternatus's height of 328'1" converts to about 99.9998 meters. So now I wanna know how rounding in the unit conversion affects it, because whether it's 99 or 100 meters drives me nuts.
Anyway, Eternatus's typing puzzles me. Or rather it's typing does. A name like "Eternatus" evokes "eternity". And it's supposed to be an evil Legendary, right? And it's said to bend Space & Time with the infinite energy.... And yet....
1. Yet it's not Dark-type, the type that has a Japanese name meaning "Evil".
2. Nor is it Ghost-type, the type for dead beings, despite appearing skeletal, & death & afterlife seeming pretty eternal, too.
3. it also has overlap with Victini (Infinite Energy, but Victini shares it, among other possible differences.) & Dialga & Palkia (Affecting space-time.), but among them, only shares Dragon type.
4. Psychic is often associated with warping reality/space (Trick Room, for example.), although, the only attacks I can think of that affect space/time are Dialga & Palkia's signatures, Roar of Time & Spacial Rend.
5. Necrozma is thematically similar, moreso as Ultra Necrozma. Being a huge source, or something connected to the power up of the game (Z Move Energy with UN, possibly Dynamax/Gigantamax with Eternatus.) & as Ultra Necrozma, also a Dragon.
6. Necrozma & Victini also have energy themes & Psychic type in common, but that may be coincidence.
7. Dragon seems to be the least confusing type of the two for Eternatus, given its appearance & probable story role. But I ponder why it wasn't partially Dark, Ghost or Psychic. (Besides Psychic being overly common on Legendaries.)
From a "meta-narrative", Poison sort of makes sense. Zacian & Zamazenta presumably have Eternatus as a common foe, & their "no equipment" forms are pure Fairy & pure Fighting types, respectively; They both gain Steel types when taking on the formes where they look as they do on the box art.
Fairy is offensively neutral to Poison/Dragon, sort of fitting for the "unbeatable sword" Legendary, as it's offense goes unresisted, even by its greatest foe, while Poison gives Eternatus SE STAB against Zamazenta without being Steel. Likewise, Poison/Dragon resists Zamazenta's STAB of Fighting. The Ultimate Shield not having the ultimate offense against its greatest foe sort of makes sense.
There's also the connection to Nihilego. Nihilego SORT OF being a Poison-type Legendary aside, its role in the story was giving dangerous, power ups, & if Dynamaxing/Gigantamaxing has negative side effects revealed in the story, Poison type makes sense in that way.
It's also only 3 turns, so it's certainly short lived like double-edged doses might be.
The other best reasons I can think of for Poison type are:
1. Poison type, other than Ghost type, comes close to "decay" and "entropy", both arguably "evil" and "eternal things", & stuff that you might consider when it's a monster that affects space-time with its energy; What is more eternal than entropy? What will come after the heat death of the universe?
2. Fairy type may be associated with Nature in general or the protection of it, as with the Tapus. Given that in mind, & Zacian being Fairy type to begin with, something representing a "natural" force that still works against protecting & preserving nature makes sense as an enemy. Even nature isn't safe from the laws of nature/physics, yet they still hinder nature.
Sorry about the super long post. It's just been on my mind a while.