By Trading:
Poliwhirl Politoed King's Rock
Slowpoke Slowking King's Rock
Onix Steelix Metal Coat
Rhydon Rhyperior Protector
Seadra Kingdra Dragon Scale
Scyther Scizor Metal Coat
Electabuzz Electivire Electrizer
Magmar Magmortar Magmarizer
Porygon Porygon2 Up-Grade
Porygon2 Porygon-Z Dubious Disc
Dusclops Dusknoir Reaper Cloth
By other conditions:
Golbat Crobat Happiness
Magneton Magnezone Mt. Coronet
Lickitung Lickilicky after Rollout learned
Chansey Blissey Happiness
Tangela Tangrowth after AncientPower learned
Eevee Espeon Happiness, Daytime
Eevee Umbreon Happiness, Nighttime
Eevee Leafeon near rock in Eterna Forest
Eevee Glaceon near rock in Route 217
Aipom Ambipom after Double Hit learned
Yanma Yanmega after AncientPower learned
Gligar Gliscor Nighttime (holding Razor Fang)
Sneasel Weavile Nighttime (holding Razor Claw)
Piloswine Mamoswine after AncientPower learned
Nosepass Probopass Mt. Coronet
By stones:
Roselia Roserade Shiny Stone
Snorunt Froslass Dawn Stone (Female)
Kirlia Gallade Dawn Stone (Male)
Murkrow Honchkrow Dusk Stone
Misdreavus Mismagius Dusk Stone
Gloom Bellossom Sun Stone
These should be all, information copied from this page:)