It looks really unlikely that it will actually become a Fire type move. a) Every legendary's sig move has matched one of their types (iirc), but the main reason is that during the trailer it was shown using it against Salamence - a Pokemon known to resist Fire, and it didn't come up saying that it was not very effective (which it would've done had it been fire).
Unless by some very strange twist it's a dual type Fire move with the other type being either Rock, Ice, Dragon or Fairy (and out of those, Rock is the only one I can see making sense) then it looks like it's not Fire.
My guess for the type of the move goes to either Steel or Psychic, just for the fact that it's one of Solgaleo's types.
Like I said previously, its mostly a error, seeing the Pokémon Wiki is still just a fan site ultimately. Still, you just might wanna wonder what GF is thinking if this was true. Again, I don't think its true. Its just a What If scenario.
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Aside that, I did some more digging and found some more convincing stuff about Solgaleo being alchemy themed as well supporting my theory of Solgaleo being inspired by the Nemean Lion.
In alchemy:
1. Solgaleo is said to be based on the alchemical symbol of "
The Lion That Devours The Sun". Goes about purifying metals, explaining the Steel type thematically.
2. Gold is ruled by the alchemic symbol of the Sun, explaining the metallic golden bits on Solgaleo's mane, which again can explain the Steel type.
3. Digestion is one of the
12 core alchemical processes and is "ruled", or "dominated", by the zodiacal sign of
Leo.
4. In alchemy, the sun is depicted in three colors, that being black, white and red, which are all present on Solgaleo's design in addition to the gold elements on its mane.
Representing the Leo Zodiac and Nemean Lion:
The Leo Zodiac Itself:
The Leo Sign represents the Nemean Lion, and is linked to the Fire element. Although Solgaleo isn't a Fire-type naturally, I expect it to learn a good number of Fire-type moves.
Solgaleo's Steel Type:
Is a reference to the Nemean Lion's impenetrable hide that is said to bounce off weaponry. Solgaleo's Ability could tie into this thematically.
Solgaleo's Star Mane:
May refer to the Leo Constellation, which is the Nemean Lion according to Greek myth. Constellations are groups of stars.
Solgaleo's Psychic Type:
Psychic is commonly associated with astrological and cosmic concepts such as the stars and space.
Solgaleo not being a Fire-type:
The Nemean Lion isn't a fire-manipulating creature. Also, appearance wise, Solgaleo barely has anything that points it to Fire-type.
Solgaleo's Golden Body when it uses its signature attack:
The Nemean Lion's body/inpenetrable hide is said to be golden in color.
Solgaleo's Emissary Role:
The Leo Zodiac, a.k.a the Nemean Lion is a servant of the Sun, which is said to be Leo's ruler, explaining Solgaleo's emissary role.
Links:
https://en.wikipedia.org/wiki/Alchemical_symbol
https://en.wikipedia.org/wiki/Digestion_(alchemy)
https://en.wikipedia.org/wiki/Suns_in_alchemy
https://en.wikipedia.org/wiki/Leo_(astrology)
https://en.wikipedia.org/wiki/Nemean_lion