Every ability

Unburden

Speed is doubled on held item loss; boost is lost if it switches, gets new item/Ability.

Standard ability
9
Pokémon
Hidden ability
7
Pokémon

In detail

If this Pokémon loses its held item for any reason, its Speed is doubled as long as it remains active, has this Ability, and is not holding an item.

Pokémon that can have it

16

Standard ability9Any one caught or hatched may have it

Hidden ability7Only one with its hidden ability has it