Cotton Spore is learned by 14 species by level and 2 by egg

Effect, in PokéAPI's words

“Lowers the target’s Speed by two stages.”

Who learns it

16 species in all, 8 of 19 games shown

Scarlet and Violet: 10 species

Species	Method	Level
Whimsicott	Level up	1
Mareep	Level up	11
Flaaffy	Level up	11
Ampharos	Level up	11
Hoppip	Level up	27
Skiploom	Level up	31
Cottonee	Level up	33
Jumpluff	Level up	35
Cacnea	Level up	46
Cacturne	Level up	49

Brilliant Diamond and Shining Pearl: 10 species

Species	Method	Level
Mareep	Level up	11
Flaaffy	Level up	11
Ampharos	Level up	11
Hoppip	Level up	34
Skiploom	Level up	40
Jumpluff	Level up	44
Cacnea	Level up	46
Cacturne	Level up	49
Roselia	Egg
Budew	Egg

Sword and Shield: 8 species

Species	Method	Level
Eldegoss	Level up	Evolve, 1
Whimsicott	Level up	1
Swirlix	Level up	24
Slurpuff	Level up	24
Cottonee	Level up	33
Maractus	Level up	40
Roselia	Egg
Budew	Egg

Ultra Sun and Ultra Moon: 15 species

Species	Method	Level
Whimsicott	Level up	1
Mareep	Level up	11
Flaaffy	Level up	11
Ampharos	Level up	11
Cottonee	Level up	17
Swirlix	Level up	17
Slurpuff	Level up	17
Maractus	Level up	18
Hoppip	Level up	34
Skiploom	Level up	40
Jumpluff	Level up	44
Cacnea	Level up	46
Cacturne	Level up	49
Roselia	Egg
Budew	Egg

Sun and Moon: 15 species

Species	Method	Level
Whimsicott	Level up	1
Mareep	Level up	11
Flaaffy	Level up	11
Ampharos	Level up	11
Cottonee	Level up	17
Swirlix	Level up	17
Slurpuff	Level up	17
Maractus	Level up	18
Hoppip	Level up	34
Skiploom	Level up	40
Jumpluff	Level up	44
Cacnea	Level up	46
Cacturne	Level up	49
Roselia	Egg
Budew	Egg

Omega Ruby and Alpha Sapphire: 15 species

Species	Method	Level
Whimsicott	Level up	1
Mareep	Level up	11
Flaaffy	Level up	11
Ampharos	Level up	11
Cottonee	Level up	17
Swirlix	Level up	17
Slurpuff	Level up	17
Maractus	Level up	18
Hoppip	Level up	34
Skiploom	Level up	40
Jumpluff	Level up	44
Cacnea	Level up	46
Cacturne	Level up	49
Roselia	Egg
Budew	Egg

X and Y: 15 species

Species	Method	Level
Whimsicott	Level up	1
Mareep	Level up	11
Flaaffy	Level up	11
Ampharos	Level up	11
Cottonee	Level up	17
Swirlix	Level up	17
Slurpuff	Level up	17
Maractus	Level up	18
Hoppip	Level up	34
Skiploom	Level up	40
Jumpluff	Level up	44
Cacnea	Level up	49
Cacturne	Level up	59
Roselia	Egg
Budew	Egg

Black 2 and White 2: 13 species

Species	Method	Level
Whimsicott	Level up	1
Mareep	Level up	11
Flaaffy	Level up	11
Ampharos	Level up	11
Cottonee	Level up	17
Maractus	Level up	18
Hoppip	Level up	34
Skiploom	Level up	40
Jumpluff	Level up	44
Cacnea	Level up	49
Cacturne	Level up	59
Roselia	Egg
Budew	Egg

Changed over time

One change since Generation II

1. Through Gen IVAccuracy 85%100% from Black and White, Generation V