Flatter is learned by 31 species by level and 11 by egg

Effect, in PokéAPI's words

“Raises the target’s Special Attack by one stage and confuses the target.”

Who learns it

42 species in all, 8 of 17 games shown

Scarlet and Violet: 31 species

Species	Method	Level
Mandibuzz	Level up	1
Morgrem	Level up	1
Grimmsnarl	Level up	1
Munkidori	Level up	1
Vullaby	Level up	6
Impidimp	Level up	8
Enamorus	Level up	10
Croagunk	Level up	12
Toxicroak	Level up	12
Shroodle	Level up	18
Grafaiai	Level up	18
Morpeko	Level up	20
Illumise	Level up	29
Oricorio	Level up	33
Gothita	Level up	40
Tinkatink	Level up	43
Tinkatuff	Level up	43
Tinkaton	Level up	43
Gothorita	Level up	46
Gothitelle	Level up	48
Mesprit	Level up	56
Fezandipiti	Level up	56
Drowzee	Egg
Totodile	Egg
Croconaw	Egg
Feraligatr	Egg
Mareep	Egg
Murkrow	Egg
Sableye	Egg
Pachirisu	Egg
Squawkabilly	Egg

Brilliant Diamond and Shining Pearl: 18 species

Species	Method	Level
Nidoqueen	Level up	1
Nidoking	Level up	1
Croagunk	Level up	12
Toxicroak	Level up	12
Illumise	Level up	29
Buneary	Level up	44
Lopunny	Level up	44
Nidoran♀	Level up	45
Nidoran♂	Level up	45
Mesprit	Level up	56
Nidorina	Level up	57
Nidorino	Level up	57
Drowzee	Egg
Totodile	Egg
Mareep	Egg
Murkrow	Egg
Sableye	Egg
Pachirisu	Egg

Sword and Shield: 23 species

Species	Method	Level
Nidoqueen	Level up	1
Nidoking	Level up	1
Mandibuzz	Level up	1
Morgrem	Level up	1
Grimmsnarl	Level up	1
Vullaby	Level up	6
Impidimp	Level up	8
Croagunk	Level up	12
Toxicroak	Level up	12
Morpeko	Level up	20
Tapu Lele	Level up	25
Gothita	Level up	40
Buneary	Level up	44
Lopunny	Level up	44
Nidoran♀	Level up	45
Nidoran♂	Level up	45
Gothorita	Level up	46
Gothitelle	Level up	48
Mesprit	Level up	56
Nidorina	Level up	57
Nidorino	Level up	57
Sableye	Egg
Woobat	Egg

Ultra Sun and Ultra Moon: 21 species

Species	Method	Level
Vullaby	Level up	19
Mandibuzz	Level up	19
Gothita	Level up	28
Gothorita	Level up	28
Gothitelle	Level up	28
Illumise	Level up	29
Nidoran♀	Level up	33
Nidoran♂	Level up	33
Nidorina	Level up	38
Nidorino	Level up	38
Croagunk	Level up	50
Tapu Lele	Level up	53
Toxicroak	Level up	62
Drowzee	Egg
Totodile	Egg
Mareep	Egg
Murkrow	Egg
Sableye	Egg
Pachirisu	Egg
Woobat	Egg
Inkay	Egg

Sun and Moon: 21 species

Species	Method	Level
Vullaby	Level up	19
Mandibuzz	Level up	19
Gothita	Level up	28
Gothorita	Level up	28
Gothitelle	Level up	28
Illumise	Level up	29
Nidoran♀	Level up	33
Nidoran♂	Level up	33
Nidorina	Level up	38
Nidorino	Level up	38
Croagunk	Level up	50
Tapu Lele	Level up	53
Toxicroak	Level up	62
Drowzee	Egg
Totodile	Egg
Mareep	Egg
Murkrow	Egg
Sableye	Egg
Pachirisu	Egg
Woobat	Egg
Inkay	Egg

Omega Ruby and Alpha Sapphire: 20 species

Species	Method	Level
Vullaby	Level up	19
Mandibuzz	Level up	19
Gothita	Level up	28
Gothorita	Level up	28
Gothitelle	Level up	28
Illumise	Level up	29
Nidoran♀	Level up	33
Nidoran♂	Level up	33
Nidorina	Level up	38
Nidorino	Level up	38
Croagunk	Level up	50
Toxicroak	Level up	62
Drowzee	Egg
Totodile	Egg
Mareep	Egg
Murkrow	Egg
Sableye	Egg
Pachirisu	Egg
Woobat	Egg
Inkay	Egg

X and Y: 20 species

Species	Method	Level
Vullaby	Level up	19
Mandibuzz	Level up	19
Gothita	Level up	28
Gothorita	Level up	28
Gothitelle	Level up	28
Illumise	Level up	29
Nidoran♀	Level up	33
Nidoran♂	Level up	33
Nidorina	Level up	38
Nidorino	Level up	38
Croagunk	Level up	50
Toxicroak	Level up	62
Drowzee	Egg
Totodile	Egg
Mareep	Egg
Murkrow	Egg
Sableye	Egg
Pachirisu	Egg
Woobat	Egg
Inkay	Egg

Black 2 and White 2: 17 species

Species	Method	Level
Vullaby	Level up	19
Mandibuzz	Level up	19
Gothita	Level up	28
Gothorita	Level up	28
Gothitelle	Level up	28
Illumise	Level up	29
Nidoran♀	Level up	33
Nidoran♂	Level up	33
Nidorina	Level up	38
Nidorino	Level up	38
Croagunk	Level up	50
Toxicroak	Level up	62
Drowzee	Egg
Mareep	Egg
Sableye	Egg
Pachirisu	Egg
Woobat	Egg