Speed is doubled on held item loss; boost is lost if it switches, gets new item/Ability.
Pokémon with Unburden
Purrloin
Liepard
Hitmonlee
Treecko
Grovyle
Sceptile
Drifloon
DrifblimUnburden
Black & White
Black & WhiteShowing Black & White. Items may require trading, an event, or downloadable content.
PurrloinLiepardHitmonleeTreeckoGrovyleSceptileDrifloonDrifblimSources: PokéAPI, PKHeX game records, Pokémon Showdown and Serebii.