Mystical Fire is learned by 9 species by level, 39 by machine, 2 by tutor and 2 by egg

Effect, in PokéAPI's words

“Has a chance to lower the target’s Special Attack by one stage.”

Who learns it

47 species in all, 8 games

Scarlet and Violet: 8 species

Species	Method	Level
Delphox	Level up	Evolve
Gardevoir	Level up	1
Mismagius	Level up	1
Armarouge	Level up	1
Enamorus	Level up	35
Flutter Mane	Level up	49
Ralts	Egg
Hatenna	Egg

Legends Arceus: 4 species

Species	Method	Level
Drifloon	Level up	25
Drifblim	Level up	25
Vulpix	Tutor
Ninetales	Tutor

Brilliant Diamond and Shining Pearl: 1 species

Species	Method	Level
Mismagius	Level up	1

Sword and Shield: 39 species

Species	Method	Level
Blacephalon	Level up, Machine	40
Charizard	Machine
Clefairy	Machine
Clefable	Machine
Vulpix	Machine
Ninetales	Machine
Ponyta	Machine
Rapidash	Machine
Mr. Mime	Machine
Flareon	Machine
Moltres	Machine
Mew	Machine
Togepi	Machine
Togetic	Machine
Ho-Oh	Machine
Gardevoir	Machine
Latias	Machine
Latios	Machine
Magmortar	Machine
Togekiss	Machine
Victini	Machine
Darmanitan	Machine
Litwick	Machine
Lampent	Machine
Chandelure	Machine
Volcarona	Machine
Reshiram	Machine
Sylveon	Machine
Pumpkaboo	Machine
Gourgeist	Machine
Diancie	Machine
Solgaleo	Machine
Centiskorch	Machine
Hatenna	Machine
Hattrem	Machine
Hatterene	Machine
Alcremie	Machine
Indeedee	Machine
Eternatus	Machine

Ultra Sun and Ultra Moon: 2 species

Species	Method	Level
Delphox	Level up	Evolve, 1
Mismagius	Level up	1

Sun and Moon: 2 species

Species	Method	Level
Delphox	Level up	Evolve, 1
Mismagius	Level up	1

Omega Ruby and Alpha Sapphire: 2 species

Species	Method	Level
Mismagius	Level up	1
Delphox	Level up	36, 75

X and Y: 1 species

Species	Method	Level
Delphox	Level up	36, 75

Changed over time

One change since Generation VI

1. Through Gen VIPower 6575 from Sun and Moon, Generation VII