Acid Armor is learned by 22 species by level and 13 by egg

Effect, in PokéAPI's words

“Raises the user’s Defense by two stages.”

Who learns it

34 species in all, 8 of 25 games shown

Scarlet and Violet: 24 species

Species	Method	Level
Sliggoo	Level up	1
Milcery	Level up	30
Alcremie	Level up	30
Phione	Level up	31
Manaphy	Level up	31
Tentacool	Level up	32
Tentacruel	Level up	34
Glimmet	Level up	41
Grimer	Level up	43
Glimmora	Level up	44
Vaporeon	Level up	45
Muk	Level up	46
Cryogonal	Level up	52
Slugma	Egg
Magcargo	Egg
Gulpin	Egg
Shellos	Egg
Solosis	Egg
Duosion	Egg
Reuniclus	Egg
Litwick	Egg
Lampent	Egg
Chandelure	Egg
Skrelp	Egg

Legends Arceus: 5 species

Species	Method	Level
Goomy	Level up	11
Phione	Level up	19
Manaphy	Level up	19
Tentacool	Level up	34
Tentacruel	Level up	34

Brilliant Diamond and Shining Pearl: 9 species

Species	Method	Level
Manaphy	Level up	31
Tentacool	Level up	32
Tentacruel	Level up	34
Grimer	Level up	43
Vaporeon	Level up	45
Muk	Level up	46
Slugma	Egg
Gulpin	Egg
Shellos	Egg

Sword and Shield: 19 species

Species	Method	Level
Jellicent	Level up	1
Accelgor	Level up	1
Shelmet	Level up	24
Milcery	Level up	30
Alcremie	Level up	30
Tentacool	Level up	32
Vanillite	Level up	32
Vanillish	Level up	32
Vanilluxe	Level up	32
Meltan	Level up	32
Melmetal	Level up	32
Tentacruel	Level up	34
Vaporeon	Level up	45
Cryogonal	Level up	52
Shellos	Egg
Solosis	Egg
Frillish	Egg
Litwick	Egg
Skrelp	Egg

Let's Go Pikachu and Let's Go Eevee: 5 species

Species	Method	Level
Vaporeon	Level up	24
Meltan	Level up	36
Melmetal	Level up	36
Grimer	Level up	42
Muk	Level up	48

Ultra Sun and Ultra Moon: 18 species

Species	Method	Level
Cryogonal	Level up	17
Vaporeon	Level up	29
Phione	Level up	31
Manaphy	Level up	31
Vanillite	Level up	31
Vanillish	Level up	31
Vanilluxe	Level up	31
Shelmet	Level up	32
Grimer	Level up	43
Muk	Level up	46
Slugma	Egg
Gulpin	Egg
Shellos	Egg
Solosis	Egg
Frillish	Egg
Litwick	Egg
Skrelp	Egg
Goomy	Egg

Sun and Moon: 18 species

Species	Method	Level
Cryogonal	Level up	17
Vaporeon	Level up	29
Phione	Level up	31
Manaphy	Level up	31
Vanillite	Level up	31
Vanillish	Level up	31
Vanilluxe	Level up	31
Shelmet	Level up	32
Grimer	Level up	43
Muk	Level up	46
Slugma	Egg
Gulpin	Egg
Shellos	Egg
Solosis	Egg
Frillish	Egg
Litwick	Egg
Skrelp	Egg
Goomy	Egg

Omega Ruby and Alpha Sapphire: 18 species

Species	Method	Level
Vaporeon	Level up	29
Cryogonal	Level up	29
Phione	Level up	31
Manaphy	Level up	31
Vanillite	Level up	31
Vanillish	Level up	31
Vanilluxe	Level up	31
Shelmet	Level up	32
Grimer	Level up	43
Muk	Level up	46
Slugma	Egg
Gulpin	Egg
Shellos	Egg
Solosis	Egg
Frillish	Egg
Litwick	Egg
Skrelp	Egg
Goomy	Egg

Changed over time

One change since Generation I

1. Through Gen VPP 4020 from X and Y, Generation VI