Swallow is learned by 25 species by level and 19 by egg

Effect, in PokéAPI's words

“Recovers 1/4 HP after one Stockpile, 1/2 HP after two Stockpiles, or full HP after three Stockpiles.”

Who learns it

43 species in all, 8 of 17 games shown

Scarlet and Violet: 33 species

Species	Method	Level
Lanturn	Level up	Evolve
Wigglytuff	Level up	1
Snorlax	Level up	1
Cramorant	Level up	1
Komala	Level up	6
Jigglypuff	Level up	12
Skwovet	Level up	15
Greedent	Level up	15
Munchlax	Level up	20
Drifloon	Level up	24
Drifblim	Level up	24
Ekans	Level up	25
Arbok	Level up	27
Pelipper	Level up	28
Gulpin	Level up	28
Swalot	Level up	30
Victreebel	Egg
Seel	Egg
Dewgong	Egg
Grimer	Egg
Koffing	Egg
Wooper	Egg
Numel	Egg
Seviper	Egg
Turtwig	Egg
Grotle	Egg
Torterra	Egg
Shellos	Egg
Hippopotas	Egg
Mareanie	Egg
Sandygast	Egg
Lechonk	Egg
Wattrel	Egg

Brilliant Diamond and Shining Pearl: 28 species

Species	Method	Level
Lanturn	Level up	Evolve, 1
Wigglytuff	Level up	1
Victreebel	Level up	1
Snorlax	Level up	1
Jigglypuff	Level up	12
Mawile	Level up	16
Munchlax	Level up	20
Drifloon	Level up	24
Drifblim	Level up	24
Ekans	Level up	25
Arbok	Level up	27
Pelipper	Level up	28
Gulpin	Level up	28
Swalot	Level up	30
Carnivine	Level up	37
Lileep	Level up	41
Cradily	Level up	43
Seel	Egg
Grimer	Egg
Koffing	Egg
Wooper	Egg
Slugma	Egg
Numel	Egg
Seviper	Egg
Spheal	Egg
Turtwig	Egg
Shellos	Egg
Hippopotas	Egg

Sword and Shield: 25 species

Species	Method	Level
Lanturn	Level up	Evolve, 1
Wigglytuff	Level up	1
Snorlax	Level up	1
Cramorant	Level up	1
Guzzlord	Level up	5
Jigglypuff	Level up	12
Skwovet	Level up	15
Greedent	Level up	15
Mawile	Level up	16
Munchlax	Level up	20
Heatmor	Level up	20
Trubbish	Level up	21
Garbodor	Level up	21
Drifloon	Level up	24
Drifblim	Level up	24
Pelipper	Level up	28
Lileep	Level up	41
Cradily	Level up	43
Koffing	Egg
Wooper	Egg
Spheal	Egg
Shellos	Egg
Hippopotas	Egg
Mareanie	Egg
Sandygast	Egg

Ultra Sun and Ultra Moon: 33 species

Species	Method	Level
Lanturn	Level up	Evolve, 1
Victreebel	Level up	1
Guzzlord	Level up	1
Komala	Level up	6
Trubbish	Level up	23
Garbodor	Level up	23
Ekans	Level up	25
Jigglypuff	Level up	25
Arbok	Level up	27
Gulpin	Level up	28
Swalot	Level up	30
Drifloon	Level up	32
Pelipper	Level up	33
Munchlax	Level up	33
Drifblim	Level up	34
Carnivine	Level up	37
Mawile	Level up	41
Lileep	Level up	46
Cradily	Level up	52
Heatmor	Level up	56
Seel	Egg
Grimer	Egg
Koffing	Egg
Wooper	Egg
Slugma	Egg
Numel	Egg
Seviper	Egg
Spheal	Egg
Turtwig	Egg
Shellos	Egg
Hippopotas	Egg
Mareanie	Egg
Sandygast	Egg

Sun and Moon: 33 species

Species	Method	Level
Lanturn	Level up	Evolve, 1
Victreebel	Level up	1
Guzzlord	Level up	1
Komala	Level up	6
Trubbish	Level up	23
Garbodor	Level up	23
Ekans	Level up	25
Jigglypuff	Level up	25
Arbok	Level up	27
Gulpin	Level up	28
Swalot	Level up	30
Drifloon	Level up	32
Pelipper	Level up	33
Munchlax	Level up	33
Drifblim	Level up	34
Carnivine	Level up	37
Mawile	Level up	41
Lileep	Level up	46
Cradily	Level up	52
Heatmor	Level up	56
Seel	Egg
Grimer	Egg
Koffing	Egg
Wooper	Egg
Slugma	Egg
Numel	Egg
Seviper	Egg
Spheal	Egg
Turtwig	Egg
Shellos	Egg
Hippopotas	Egg
Mareanie	Egg
Sandygast	Egg

Omega Ruby and Alpha Sapphire: 28 species

Species	Method	Level
Victreebel	Level up	1
Trubbish	Level up	23
Garbodor	Level up	23
Ekans	Level up	25
Arbok	Level up	27
Lanturn	Level up	27
Gulpin	Level up	28
Swalot	Level up	30
Drifloon	Level up	32
Pelipper	Level up	33
Munchlax	Level up	33
Drifblim	Level up	34
Carnivine	Level up	37
Mawile	Level up	41
Lileep	Level up	46
Heatmor	Level up	50
Cradily	Level up	52
Seel	Egg
Grimer	Egg
Koffing	Egg
Wooper	Egg
Slugma	Egg
Numel	Egg
Seviper	Egg
Spheal	Egg
Turtwig	Egg
Shellos	Egg
Hippopotas	Egg

X and Y: 28 species

Species	Method	Level
Victreebel	Level up	1
Cradily	Level up	1, 66
Trubbish	Level up	23
Garbodor	Level up	23
Ekans	Level up	25
Arbok	Level up	27
Lanturn	Level up	27
Drifloon	Level up	32
Munchlax	Level up	33
Drifblim	Level up	34
Carnivine	Level up	37
Pelipper	Level up	39
Gulpin	Level up	39
Swalot	Level up	45
Mawile	Level up	50
Heatmor	Level up	50
Lileep	Level up	57
Seel	Egg
Grimer	Egg
Koffing	Egg
Wooper	Egg
Slugma	Egg
Numel	Egg
Seviper	Egg
Spheal	Egg
Turtwig	Egg
Shellos	Egg
Hippopotas	Egg

Black 2 and White 2: 28 species

Species	Method	Level
Victreebel	Level up	1
Trubbish	Level up	23
Garbodor	Level up	23
Ekans	Level up	25
Arbok	Level up	27
Lanturn	Level up	27
Drifloon	Level up	32
Munchlax	Level up	33
Drifblim	Level up	34
Carnivine	Level up	37
Pelipper	Level up	39
Gulpin	Level up	39
Swalot	Level up	45
Mawile	Level up	51
Heatmor	Level up	56
Lileep	Level up	57
Cradily	Level up	66
Seel	Egg
Grimer	Egg
Koffing	Egg
Wooper	Egg
Slugma	Egg
Numel	Egg
Seviper	Egg
Spheal	Egg
Turtwig	Egg
Shellos	Egg
Hippopotas	Egg