Meteor Mash is learned by 6 species by level and 1 by egg

Effect, in PokéAPI's words

“Has a chance to raise the user’s Attack by one stage.”

Who learns it

7 species in all, 8 of 17 games shown

Scarlet and Violet: 6 species

Species	Method	Level
Clefable	Level up	1
Clefairy	Level up	32
Lucario	Level up	48
Jirachi	Level up	49
Metang	Level up	50
Metagross	Level up	52

Brilliant Diamond and Shining Pearl: 7 species

Species	Method	Level
Clefable	Level up	1
Clefairy	Level up	32
Lucario	Level up	48
Jirachi	Level up	49
Metang	Level up	50
Metagross	Level up	52
Riolu	Egg

Sword and Shield: 6 species

Species	Method	Level
Clefable	Level up	1
Clefairy	Level up	32
Lucario	Level up	48
Jirachi	Level up	49
Metang	Level up	50
Metagross	Level up	52

Ultra Sun and Ultra Moon: 4 species

Species	Method	Level
Metang	Level up	44
Metagross	Level up	44
Clefairy	Level up	50
Riolu	Egg

Sun and Moon: 3 species

Species	Method	Level
Metang	Level up	44
Metagross	Level up	44
Clefairy	Level up	50

Omega Ruby and Alpha Sapphire: 3 species

Species	Method	Level
Metang	Level up	44
Metagross	Level up	44
Clefairy	Level up	50

X and Y: 3 species

Species	Method	Level
Metang	Level up	44
Metagross	Level up	44
Clefairy	Level up	50

Black 2 and White 2: 3 species

Species	Method	Level
Metang	Level up	44
Metagross	Level up	44
Clefairy	Level up	52

Changed over time

2 changes since Generation III

1. Through Gen VPower 10090 from X and Y, Generation VI
2. Through Gen VAccuracy 85%90% from X and Y, Generation VI