Pokémon Crystal Unlockables

Ingame Trades
Some NPC trainers will trade you a pokemon if you have the one they request.
PokemonObtain by:
OnixGo to the house at the far right of Violet City. Then, talk to the boy. He wants a Bellsprout, so select Bellsprout and you will get Onix.
MachopA man on the 5th floor of the Goldenrod Department Story will trade you this. He wants an Abra, so select Abra and you will get Machop.
VoltrobBefore you enter lighthouse across cianwood there 2 house above that lighthouse the left one will trade a krabby for a voltrob
DodrioIn the easternmost house of Blackthorn City a lady will trade you a Dodrio for a female Dragonair.
AerodactylThere is a girl walking around the grass on Route 14 that will trade you an Aerodactyl for a Chansey. You have a small chance but you can find Chansey in this area.
XatoThere is a man in the Pokemon Center that will trade you a Xatu for a Haunter.
MagnetonIn the Power Plant on Route 10 there is a scientist that will trade you a Magneton for a Dugtrio.