Memento is learned by 30 species by level and 24 by egg

Effect, in PokéAPI's words

“Lowers the target’s Attack and Special Attack by two stages. User faints.”

Who learns it

52 species in all, 8 of 17 games shown

Scarlet and Violet: 50 species

Species	Method	Level
Whimsicott	Level up, Egg	1
Chandelure	Level up	1
Bombirdier	Level up	1
Pecharunt	Level up	1
Flutter Mane	Level up	21
Spiritomb	Level up	30
Stunky	Level up	33
Skuntank	Level up	33
Tatsugiri	Level up	34
Hoppip	Level up	38
Skiploom	Level up	44
Shellos	Level up	45
Grimer	Level up	48
Koffing	Level up	48
Gastrodon	Level up	53
Sinistea	Level up	54
Polteageist	Level up	54
Poltchageist	Level up	54
Sinistcha	Level up	54
Jumpluff	Level up	55
Weezing	Level up	56
Litwick	Level up	56
Muk	Level up	57
Lampent	Level up	64
Ting-Lu	Level up	65
Latios	Level up	70
Gholdengo	Level up	70
Chi-Yu	Level up	70
Uxie	Level up	77
Vulpix	Egg
Ninetales	Egg
Diglett	Egg
Misdreavus	Egg
Slugma	Egg
Magcargo	Egg
Delibird	Egg
Ralts	Egg
Duskull	Egg
Dusclops	Egg
Drifloon	Egg
Dusknoir	Egg
Cottonee	Egg
Zorua	Egg
Pincurchin	Egg
Tarountula	Egg
Smoliv	Egg
Rellor	Egg
Wiglett	Egg
Glimmet	Egg
Greavard	Egg

Brilliant Diamond and Shining Pearl: 22 species

Species	Method	Level
Spiritomb	Level up	30
Stunky	Level up	33
Skuntank	Level up	33
Shellos	Level up, Egg	45
Grimer	Level up	48
Koffing	Level up	48
Hoppip	Level up	49
Gastrodon	Level up	53
Weezing	Level up	56
Muk	Level up	57
Skiploom	Level up	60
Jumpluff	Level up	69
Latios	Level up	70
Uxie	Level up	77
Vulpix	Egg
Diglett	Egg
Misdreavus	Egg
Slugma	Egg
Delibird	Egg
Ralts	Egg
Duskull	Egg
Drifloon	Egg

Sword and Shield: 26 species

Species	Method	Level
Whimsicott	Level up	1
Chandelure	Level up	1
Spiritomb	Level up	30
Stunky	Level up	33
Skuntank	Level up	33
Shellos	Level up	45
Koffing	Level up	48
Gastrodon	Level up	53
Sinistea	Level up	54
Polteageist	Level up	54
Weezing	Level up	56
Litwick	Level up	56
Pyukumuku	Level up	60
Lampent	Level up	64
Latios	Level up	70
Uxie	Level up	77
Vulpix	Egg
Diglett	Egg
Delibird	Egg
Ralts	Egg
Duskull	Egg
Drifloon	Egg
Cottonee	Egg
Yamask	Egg
Zorua	Egg
Pincurchin	Egg

Ultra Sun and Ultra Moon: 25 species

Species	Method	Level
Latios	Level up	1, 61
Uxie	Level up	1, 76
Stunky	Level up	33
Skuntank	Level up	33
Litwick	Level up	33
Lampent	Level up	33
Spiritomb	Level up	43
Koffing	Level up	45
Grimer	Level up	48
Hoppip	Level up	49
Pyukumuku	Level up	49
Muk	Level up	57
Weezing	Level up	57
Skiploom	Level up	60
Jumpluff	Level up	69
Diglett	Egg
Misdreavus	Egg
Slugma	Egg
Ralts	Egg
Duskull	Egg
Shellos	Egg
Drifloon	Egg
Cottonee	Egg
Yamask	Egg
Zorua	Egg

Sun and Moon: 25 species

Species	Method	Level
Latios	Level up	1, 61
Uxie	Level up	1, 76
Stunky	Level up	33
Skuntank	Level up	33
Litwick	Level up	33
Lampent	Level up	33
Spiritomb	Level up	43
Koffing	Level up	45
Grimer	Level up	48
Hoppip	Level up	49
Pyukumuku	Level up	49
Muk	Level up	57
Weezing	Level up	57
Skiploom	Level up	60
Jumpluff	Level up	69
Diglett	Egg
Misdreavus	Egg
Slugma	Egg
Ralts	Egg
Duskull	Egg
Shellos	Egg
Drifloon	Egg
Cottonee	Egg
Yamask	Egg
Zorua	Egg

Omega Ruby and Alpha Sapphire: 24 species

Species	Method	Level
Latios	Level up	1, 61
Uxie	Level up	1, 76
Litwick	Level up	33
Lampent	Level up	33
Stunky	Level up	43
Spiritomb	Level up	43
Koffing	Level up	45
Grimer	Level up	48
Hoppip	Level up	49
Skuntank	Level up	51
Muk	Level up	57
Weezing	Level up	57
Skiploom	Level up	60
Jumpluff	Level up	69
Diglett	Egg
Misdreavus	Egg
Slugma	Egg
Ralts	Egg
Duskull	Egg
Shellos	Egg
Drifloon	Egg
Cottonee	Egg
Yamask	Egg
Zorua	Egg

X and Y: 24 species

Species	Method	Level
Latios	Level up	1, 85
Uxie	Level up	1, 76
Litwick	Level up	33
Lampent	Level up	33
Stunky	Level up	43
Spiritomb	Level up	43
Koffing	Level up	45
Grimer	Level up	48
Hoppip	Level up	49
Skuntank	Level up	51
Weezing	Level up	54
Muk	Level up	57
Skiploom	Level up	60
Jumpluff	Level up	69
Diglett	Egg
Misdreavus	Egg
Slugma	Egg
Ralts	Egg
Duskull	Egg
Shellos	Egg
Drifloon	Egg
Cottonee	Egg
Yamask	Egg
Zorua	Egg

Black 2 and White 2: 24 species

Species	Method	Level
Litwick	Level up	33
Lampent	Level up	33
Stunky	Level up	43
Spiritomb	Level up	43
Koffing	Level up	45
Grimer	Level up	48
Hoppip	Level up	49
Skuntank	Level up	51
Weezing	Level up	54
Muk	Level up	57
Skiploom	Level up	60
Jumpluff	Level up	69
Uxie	Level up	76
Latios	Level up	85
Diglett	Egg
Misdreavus	Egg
Slugma	Egg
Ralts	Egg
Duskull	Egg
Shellos	Egg
Drifloon	Egg
Cottonee	Egg
Yamask	Egg
Zorua	Egg