Defog is learned by 12 species by level, 69 by machine, 142 by tutor and 28 by egg

Effect, in PokéAPI's words

“Lowers the target’s evasion by one stage. Removes field effects from the enemy field.”

Who learns it

150 species in all, 8 of 11 games shown

Scarlet and Violet: 29 species

Species	Method	Level
Giratina	Level up	1
Swanna	Level up	1
Ducklett	Level up	6
Hoothoot	Level up	15
Frosmoth	Level up	16
Rufflet	Level up	60
Vullaby	Level up	60
Braviary	Level up	64
Mandibuzz	Level up	64
Scyther	Egg
Seedot	Egg
Nuzleaf	Egg
Shiftry	Egg
Swablu	Egg
Drifloon	Egg
Timburr	Egg
Gurdurr	Egg
Conkeldurr	Egg
Fletchling	Egg
Hawlucha	Egg
Noibat	Egg
Rowlet	Egg
Dartrix	Egg
Decidueye	Egg
Oricorio	Egg
Fomantis	Egg
Rookidee	Egg
Corviknight	Egg
Cramorant	Egg

Brilliant Diamond and Shining Pearl: 67 species

Species	Method	Level
Giratina	Level up, Machine	1
Butterfree	Machine
Beedrill	Machine
Pidgey	Machine
Pidgeotto	Machine
Pidgeot	Machine
Spearow	Machine
Fearow	Machine
Zubat	Machine
Golbat	Machine
Venomoth	Machine
Farfetch’d	Machine
Scyther	Machine
Aerodactyl	Machine
Articuno	Machine
Zapdos	Machine
Moltres	Machine
Dragonite	Machine
Mew	Machine
Hoothoot	Machine
Noctowl	Machine
Crobat	Machine
Togetic	Machine
Xatu	Machine
Yanma	Machine
Murkrow	Machine
Gligar	Machine
Scizor	Machine
Delibird	Machine
Mantine	Machine
Skarmory	Machine
Lugia	Machine
Ho-Oh	Machine
Beautifly	Machine
Dustox	Machine
Shiftry	Machine
Taillow	Machine
Swellow	Machine
Wingull	Machine
Pelipper	Machine
Masquerain	Machine
Ninjask	Machine
Vibrava	Machine
Flygon	Machine
Tropius	Machine
Salamence	Machine
Latias	Machine
Latios	Machine
Piplup	Machine
Prinplup	Machine
Empoleon	Machine
Starly	Machine
Staravia	Machine
Staraptor	Machine
Mothim	Machine
Vespiquen	Machine
Drifloon	Machine
Drifblim	Machine
Honchkrow	Machine
Stunky	Machine
Skuntank	Machine
Chatot	Machine
Finneon	Machine
Lumineon	Machine
Togekiss	Machine
Yanmega	Machine
Gliscor	Machine

Sword and Shield: 26 species

Species	Method	Level
Giratina	Level up	1
Frosmoth	Level up	16
Tapu Fini	Level up	30
Sirfetch’d	Level up	35
Kartana	Level up	50
Rufflet	Level up	60
Vullaby	Level up	60
Braviary	Level up	64
Mandibuzz	Level up	64
Zubat	Egg
Scyther	Egg
Hoothoot	Egg
Seedot	Egg
Swablu	Egg
Drifloon	Egg
Pidove	Egg
Timburr	Egg
Archen	Egg
Emolga	Egg
Fletchling	Egg
Hawlucha	Egg
Noibat	Egg
Rowlet	Egg
Fomantis	Egg
Rookidee	Egg
Cramorant	Egg

Ultra Sun and Ultra Moon: 142 species

Species	Method	Level
Swanna	Level up, Tutor	1, 6
Kartana	Level up, Tutor	1
Ducklett	Level up, Tutor	6
Rufflet	Level up, Tutor	32
Braviary	Level up, Tutor	32
Vullaby	Level up, Tutor	32
Mandibuzz	Level up, Tutor	32
Tapu Fini	Level up, Tutor	38
Charizard	Tutor
Butterfree	Tutor
Beedrill	Tutor
Pidgey	Tutor, Egg
Pidgeotto	Tutor
Pidgeot	Tutor
Spearow	Tutor
Fearow	Tutor
Zubat	Tutor, Egg
Golbat	Tutor
Venomoth	Tutor
Farfetch’d	Tutor
Scyther	Tutor, Egg
Aerodactyl	Tutor
Articuno	Tutor
Zapdos	Tutor
Moltres	Tutor
Dragonite	Tutor
Mew	Tutor
Hoothoot	Tutor, Egg
Noctowl	Tutor
Ledian	Tutor
Crobat	Tutor
Togetic	Tutor
Xatu	Tutor
Yanma	Tutor
Murkrow	Tutor
Gligar	Tutor
Scizor	Tutor
Delibird	Tutor
Mantine	Tutor
Skarmory	Tutor
Lugia	Tutor
Ho-Oh	Tutor
Torchic	Tutor
Combusken	Tutor
Blaziken	Tutor
Beautifly	Tutor
Dustox	Tutor
Seedot	Tutor, Egg
Nuzleaf	Tutor
Shiftry	Tutor
Taillow	Tutor, Egg
Swellow	Tutor
Wingull	Tutor
Pelipper	Tutor
Masquerain	Tutor
Ninjask	Tutor
Volbeat	Tutor
Illumise	Tutor
Vibrava	Tutor
Flygon	Tutor
Swablu	Tutor
Altaria	Tutor
Castform	Tutor
Tropius	Tutor
Chimecho	Tutor
Salamence	Tutor
Latias	Tutor
Latios	Tutor
Rayquaza	Tutor
Piplup	Tutor
Prinplup	Tutor
Empoleon	Tutor
Starly	Tutor
Staravia	Tutor
Staraptor	Tutor
Mothim	Tutor
Vespiquen	Tutor
Drifloon	Tutor, Egg
Drifblim	Tutor
Honchkrow	Tutor
Stunky	Tutor
Skuntank	Tutor
Chatot	Tutor, Egg
Carnivine	Tutor
Finneon	Tutor
Lumineon	Tutor
Togekiss	Tutor
Yanmega	Tutor
Gliscor	Tutor
Rotom	Tutor
Giratina	Tutor
Arceus	Tutor
Snivy	Tutor
Servine	Tutor
Serperior	Tutor
Pidove	Tutor
Tranquill	Tutor
Unfezant	Tutor
Woobat	Tutor
Swoobat	Tutor
Cottonee	Tutor
Whimsicott	Tutor
Sigilyph	Tutor
Archen	Tutor, Egg
Archeops	Tutor
Emolga	Tutor
Cryogonal	Tutor
Hydreigon	Tutor
Volcarona	Tutor
Tornadus	Tutor
Thundurus	Tutor
Reshiram	Tutor
Zekrom	Tutor
Landorus	Tutor
Fletchling	Tutor
Fletchinder	Tutor
Talonflame	Tutor
Vivillon	Tutor
Florges	Tutor
Hawlucha	Tutor
Klefki	Tutor
Noibat	Tutor
Noivern	Tutor
Xerneas	Tutor
Yveltal	Tutor
Volcanion	Tutor
Rowlet	Tutor, Egg
Dartrix	Tutor
Decidueye	Tutor
Pikipek	Tutor
Trumbeak	Tutor
Toucannon	Tutor
Oricorio	Tutor
Cutiefly	Tutor
Ribombee	Tutor
Fomantis	Tutor, Egg
Lurantis	Tutor
Comfey	Tutor
Silvally	Tutor
Drampa	Tutor
Tapu Koko	Tutor
Lunala	Tutor

Sun and Moon: 19 species

Species	Method	Level
Swanna	Level up	1, 6
Kartana	Level up	1
Ducklett	Level up	6
Rufflet	Level up	32
Braviary	Level up	32
Vullaby	Level up	32
Mandibuzz	Level up	32
Tapu Fini	Level up	38
Pidgey	Egg
Zubat	Egg
Scyther	Egg
Hoothoot	Egg
Seedot	Egg
Taillow	Egg
Drifloon	Egg
Chatot	Egg
Archen	Egg
Rowlet	Egg
Fomantis	Egg

Omega Ruby and Alpha Sapphire: 15 species

Species	Method	Level
Swanna	Level up	1, 6
Ducklett	Level up	6
Rufflet	Level up	32
Braviary	Level up	32
Vullaby	Level up	32
Mandibuzz	Level up	32
Pidgey	Egg
Zubat	Egg
Scyther	Egg
Hoothoot	Egg
Seedot	Egg
Taillow	Egg
Drifloon	Egg
Chatot	Egg
Archen	Egg

X and Y: 15 species

Species	Method	Level
Swanna	Level up	1, 6
Ducklett	Level up	6
Rufflet	Level up	32
Braviary	Level up	32
Vullaby	Level up	32
Mandibuzz	Level up	32
Pidgey	Egg
Zubat	Egg
Scyther	Egg
Hoothoot	Egg
Seedot	Egg
Taillow	Egg
Drifloon	Egg
Chatot	Egg
Archen	Egg

Black 2 and White 2: 15 species

Species	Method	Level
Swanna	Level up	1, 6
Ducklett	Level up	6
Rufflet	Level up	32
Braviary	Level up	32
Vullaby	Level up	32
Mandibuzz	Level up	32
Pidgey	Egg
Zubat	Egg
Scyther	Egg
Hoothoot	Egg
Seedot	Egg
Taillow	Egg
Drifloon	Egg
Chatot	Egg
Archen	Egg