Flail is learned by 87 species by level and 57 by egg

Effect, in PokéAPI's words

“Inflicts more damage when the user has less HP remaining, with a maximum of 200 power.”

Who learns it

138 species in all, 6 of 19 games shown

Scarlet and Violet: 95 species

Species	Method	Level
Gyarados	Level up	1
Snorlax	Level up	1
Sudowoodo	Level up	1
Dunsparce	Level up	1
Piloswine	Level up	1
Lombre	Level up	1
Milotic	Level up	1
Mamoswine	Level up	1
Clawitzer	Level up	1
Steenee	Level up	1
Tsareena	Level up	1
Toxel	Level up	1
Toxtricity	Level up	1
Dudunsparce	Level up	1
Bonsly	Level up	4
Phanpy	Level up	6
Basculin	Level up	8
Basculegion	Level up	8
Swinub	Level up	10
Clauncher	Level up	10
Dondozo	Level up	10
Fletchling	Level up	15
Fletchinder	Level up	15
Talonflame	Level up	15
Carbink	Level up	15
Lotad	Level up	16
Smoliv	Level up	16
Dolliv	Level up	16
Arboliva	Level up	16
Goomy	Level up	20
Sliggoo	Level up	20
Goodra	Level up	20
Diancie	Level up	21
Totodile	Level up	22
Croconaw	Level up	24
Feraligatr	Level up	24
Cubchoo	Level up	24
Beartic	Level up	24
Bounsweet	Level up	24
Magikarp	Level up	25
Feebas	Level up	25
Tadbulb	Level up	25
Bellibolt	Level up	25
Cetoddle	Level up	25
Cetitan	Level up	25
Luvdisc	Level up	26
Komala	Level up	26
Slakoth	Level up	33
Chinchou	Level up	36
Klawf	Level up	37
Slaking	Level up	39
Lanturn	Level up	42
Sewaddle	Level up	43
Swadloon	Level up	43
Munchlax	Level up	44
Uxie	Level up	70
Squirtle	Egg
Wartortle	Egg
Blastoise	Egg
Sandshrew	Egg
Sandslash	Egg
Vulpix	Egg
Ninetales	Egg
Oddish	Egg
Gloom	Egg
Vileplume	Egg
Meowth	Egg
Geodude	Egg
Graveler	Egg
Golem	Egg
Doduo	Egg
Dodrio	Egg
Horsea	Egg
Seadra	Egg
Eevee	Egg
Chikorita	Egg
Bayleef	Egg
Meganium	Egg
Pichu	Egg
Bellossom	Egg
Pineco	Egg
Qwilfish	Egg
Heracross	Egg
Kingdra	Egg
Ludicolo	Egg
Torkoal	Egg
Trapinch	Egg
Vibrava	Egg
Flygon	Egg
Zangoose	Egg
Barboach	Egg
Pachirisu	Egg
Finneon	Egg
Minccino	Egg
Cinccino	Egg

Brilliant Diamond and Shining Pearl: 69 species

Species	Method	Level
Gyarados	Level up	1
Snorlax	Level up	1
Sudowoodo	Level up	1
Dunsparce	Level up	1
Piloswine	Level up	1
Donphan	Level up	1
Lombre	Level up	1
Ludicolo	Level up	1
Vigoroth	Level up	1
Milotic	Level up	1
Mamoswine	Level up	1
Bonsly	Level up	4
Phanpy	Level up	6
Swinub	Level up	10
Lotad	Level up, Egg	16
Totodile	Level up	22
Croconaw	Level up	24
Feraligatr	Level up	24
Magikarp	Level up	25
Feebas	Level up	25
Luvdisc	Level up	26
Finneon	Level up, Egg	26
Lumineon	Level up	26
Krabby	Level up, Egg	29
Goldeen	Level up	30
Seaking	Level up	30
Corsola	Level up	30
Zigzagoon	Level up	30
Kingler	Level up	31
Slakoth	Level up	33
Chinchou	Level up, Egg	36
Linoone	Level up	38
Wormadam	Level up	38
Slaking	Level up	39
Relicanth	Level up	40
Lanturn	Level up	42
Munchlax	Level up	44
Spinda	Level up	50
Uxie	Level up	70
Squirtle	Egg
Sandshrew	Egg
Vulpix	Egg
Oddish	Egg
Paras	Egg
Meowth	Egg
Geodude	Egg
Farfetch’d	Egg
Doduo	Egg
Onix	Egg
Tangela	Egg
Horsea	Egg
Pinsir	Egg
Eevee	Egg
Kabuto	Egg
Chikorita	Egg
Pichu	Egg
Pineco	Egg
Qwilfish	Egg
Heracross	Egg
Remoraid	Egg
Nincada	Egg
Torkoal	Egg
Trapinch	Egg
Zangoose	Egg
Barboach	Egg
Piplup	Egg
Pachirisu	Egg
Buneary	Egg
Glameow	Egg

Sword and Shield: 83 species

Species	Method	Level
Gyarados	Level up	1
Snorlax	Level up	1
Sudowoodo	Level up	1
Dunsparce	Level up	1
Piloswine	Level up	1
Lombre	Level up	1
Ludicolo	Level up	1
Milotic	Level up	1
Mamoswine	Level up	1
Escavalier	Level up	1
Clawitzer	Level up	1
Steenee	Level up	1
Tsareena	Level up	1
Toxel	Level up	1
Toxtricity	Level up	1
Bonsly	Level up	4
Basculin	Level up	8
Swinub	Level up	10
Clauncher	Level up	10
Bunnelby	Level up	15
Diggersby	Level up	15
Fletchling	Level up	15
Fletchinder	Level up	15
Talonflame	Level up	15
Carbink	Level up	15
Lotad	Level up	16
Dwebble	Level up	16
Crustle	Level up	16
Turtonator	Level up	16
Goomy	Level up	20
Sliggoo	Level up	20
Goodra	Level up	20
Spritzee	Level up	21
Aromatisse	Level up	21
Diancie	Level up	21
Tympole	Level up	24
Palpitoad	Level up	24
Seismitoad	Level up	24
Karrablast	Level up	24
Cubchoo	Level up	24
Beartic	Level up	24
Bounsweet	Level up	24
Magikarp	Level up	25
Feebas	Level up	25
Stufful	Level up	28
Krabby	Level up	29
Goldeen	Level up	30
Seaking	Level up	30
Corsola	Level up	30
Zigzagoon	Level up	30
Bewear	Level up	30
Kingler	Level up	31
Chinchou	Level up	36
Linoone	Level up	38
Relicanth	Level up	40
Lanturn	Level up	42
Munchlax	Level up	44
Stunfisk	Level up	50
Uxie	Level up	70
Squirtle	Egg
Sandshrew	Egg
Vulpix	Egg
Oddish	Egg
Meowth	Egg
Farfetch’d	Egg
Onix	Egg
Tangela	Egg
Horsea	Egg
Pinsir	Egg
Eevee	Egg
Kabuto	Egg
Pichu	Egg
Qwilfish	Egg
Heracross	Egg
Remoraid	Egg
Nincada	Egg
Torkoal	Egg
Trapinch	Egg
Barboach	Egg
Buneary	Egg
Minccino	Egg
Durant	Egg
Togedemaru	Egg

Ultra Sun and Ultra Moon: 90 species

Species	Method	Level
Sudowoodo	Level up	1, 5
Relicanth	Level up	1
Uxie	Level up	1, 61
Stunfisk	Level up	1, 55
Bonsly	Level up	5
Phanpy	Level up	6
Goldeen	Level up	13
Seaking	Level up	13
Fletchling	Level up	16
Fletchinder	Level up	16
Talonflame	Level up	16
Clauncher	Level up	16
Clawitzer	Level up	16
Turtonator	Level up	17
Stufful	Level up	19
Bewear	Level up	19
Totodile	Level up	22
Croconaw	Level up	24
Feraligatr	Level up	24
Luvdisc	Level up	26
Komala	Level up	26
Goomy	Level up	28
Sliggoo	Level up	28
Goodra	Level up	28
Zigzagoon	Level up	29
Bunnelby	Level up	29
Bounsweet	Level up	29
Magikarp	Level up	30
Feebas	Level up	30
Chinchou	Level up, Egg	31
Diancie	Level up	31
Diggersby	Level up	32
Lanturn	Level up	33
Slakoth	Level up	33
Tympole	Level up	34
Basculin	Level up	34
Carbink	Level up	35
Cubchoo	Level up	36
Beartic	Level up	36
Palpitoad	Level up	37
Wormadam	Level up	38
Spritzee	Level up	38
Aromatisse	Level up	38
Slaking	Level up	39
Seismitoad	Level up	39
Swinub	Level up	40
Dwebble	Level up	41
Torkoal	Level up	42
Sewaddle	Level up	43
Krabby	Level up, Egg	45
Karrablast	Level up	49
Corsola	Level up	50
Spinda	Level up	50
Crustle	Level up	50
Dunsparce	Level up	51
Kingler	Level up	63
Squirtle	Egg
Sandshrew	Egg
Vulpix	Egg
Oddish	Egg
Paras	Egg
Meowth	Egg
Geodude	Egg
Farfetch’d	Egg
Doduo	Egg
Onix	Egg
Tangela	Egg
Horsea	Egg
Pinsir	Egg
Eevee	Egg
Kabuto	Egg
Chikorita	Egg
Pichu	Egg
Pineco	Egg
Qwilfish	Egg
Heracross	Egg
Remoraid	Egg
Lotad	Egg
Trapinch	Egg
Zangoose	Egg
Barboach	Egg
Piplup	Egg
Pachirisu	Egg
Buneary	Egg
Glameow	Egg
Finneon	Egg
Patrat	Egg
Tirtouga	Egg
Minccino	Egg
Togedemaru	Egg

Sun and Moon: 90 species

Species	Method	Level
Sudowoodo	Level up	1, 5
Relicanth	Level up	1
Uxie	Level up	1, 61
Stunfisk	Level up	1, 55
Bonsly	Level up	5
Phanpy	Level up	6
Goldeen	Level up	13
Seaking	Level up	13
Fletchling	Level up	16
Fletchinder	Level up	16
Talonflame	Level up	16
Clauncher	Level up	16
Clawitzer	Level up	16
Turtonator	Level up	17
Stufful	Level up	19
Bewear	Level up	19
Totodile	Level up	22
Croconaw	Level up	24
Feraligatr	Level up	24
Luvdisc	Level up	26
Komala	Level up	26
Goomy	Level up	28
Sliggoo	Level up	28
Goodra	Level up	28
Zigzagoon	Level up	29
Bunnelby	Level up	29
Bounsweet	Level up	29
Magikarp	Level up	30
Feebas	Level up	30
Chinchou	Level up, Egg	31
Diancie	Level up	31
Diggersby	Level up	32
Lanturn	Level up	33
Slakoth	Level up	33
Tympole	Level up	34
Basculin	Level up	34
Carbink	Level up	35
Cubchoo	Level up	36
Beartic	Level up	36
Palpitoad	Level up	37
Wormadam	Level up	38
Spritzee	Level up	38
Aromatisse	Level up	38
Slaking	Level up	39
Seismitoad	Level up	39
Swinub	Level up	40
Dwebble	Level up	41
Torkoal	Level up	42
Sewaddle	Level up	43
Krabby	Level up, Egg	45
Dunsparce	Level up	48
Karrablast	Level up	49
Corsola	Level up	50
Spinda	Level up	50
Crustle	Level up	50
Kingler	Level up	63
Squirtle	Egg
Sandshrew	Egg
Vulpix	Egg
Oddish	Egg
Paras	Egg
Meowth	Egg
Geodude	Egg
Farfetch’d	Egg
Doduo	Egg
Onix	Egg
Tangela	Egg
Horsea	Egg
Pinsir	Egg
Eevee	Egg
Kabuto	Egg
Chikorita	Egg
Pichu	Egg
Pineco	Egg
Qwilfish	Egg
Heracross	Egg
Remoraid	Egg
Lotad	Egg
Trapinch	Egg
Zangoose	Egg
Barboach	Egg
Piplup	Egg
Pachirisu	Egg
Buneary	Egg
Glameow	Egg
Finneon	Egg
Patrat	Egg
Tirtouga	Egg
Minccino	Egg
Togedemaru	Egg

Omega Ruby and Alpha Sapphire: 83 species

Species	Method	Level
Sudowoodo	Level up	1, 5
Uxie	Level up	1, 61
Basculin	Level up	1, 46
Stunfisk	Level up	1, 55
Bonsly	Level up	5
Phanpy	Level up	6
Goldeen	Level up	13
Seaking	Level up	13
Fletchling	Level up	16
Fletchinder	Level up	16
Talonflame	Level up	16
Clauncher	Level up	16
Clawitzer	Level up	16
Totodile	Level up	22
Croconaw	Level up	24
Feraligatr	Level up	24
Luvdisc	Level up	27
Goomy	Level up	28
Sliggoo	Level up	28
Goodra	Level up	28
Zigzagoon	Level up	29
Bunnelby	Level up	29
Magikarp	Level up	30
Feebas	Level up	30
Chinchou	Level up, Egg	31
Diggersby	Level up	31
Lanturn	Level up	33
Slakoth	Level up	33
Tympole	Level up	34
Carbink	Level up	35
Diancie	Level up	35
Cubchoo	Level up	36
Beartic	Level up	36
Palpitoad	Level up	37
Wormadam	Level up	38
Spritzee	Level up	38
Aromatisse	Level up	38
Slaking	Level up	39
Seismitoad	Level up	39
Swinub	Level up	40
Dwebble	Level up	41
Torkoal	Level up	42
Sewaddle	Level up	43
Krabby	Level up, Egg	45
Dunsparce	Level up	49
Karrablast	Level up	49
Corsola	Level up	50
Spinda	Level up	50
Crustle	Level up	50
Kingler	Level up	63
Squirtle	Egg
Sandshrew	Egg
Vulpix	Egg
Oddish	Egg
Paras	Egg
Meowth	Egg
Geodude	Egg
Farfetch’d	Egg
Doduo	Egg
Onix	Egg
Tangela	Egg
Horsea	Egg
Pinsir	Egg
Eevee	Egg
Kabuto	Egg
Chikorita	Egg
Pichu	Egg
Pineco	Egg
Qwilfish	Egg
Heracross	Egg
Remoraid	Egg
Lotad	Egg
Trapinch	Egg
Zangoose	Egg
Barboach	Egg
Piplup	Egg
Pachirisu	Egg
Buneary	Egg
Glameow	Egg
Finneon	Egg
Patrat	Egg
Tirtouga	Egg
Minccino	Egg