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On the ground floor you will find these pokémon:
Sandslash-Rare
Gloom-Rare
Parasect-Rare
Venomoth-Rare
Weepinbell-Rare
Golbat-Common
Gravelar-Common
On the 1st floor...
sandslash-rare
gloom-rare
weepinbell-rare
rhyhorn-rare
rhydon-rare
ditto-rare
golbat-common
gravelar-common
3rd floor
lickitung-rare
rhyhorn-rare
rhydon-rare
chansey-rare
ditto-rare
golbat-common
gravelar-common
Mewtwo-only one at the end of the dungeon.
Rare/common only relate to the unknown dungeon.
Poliwag - Pallet town, Viridian City, Route 22
Poliwhirl - Celadon City
Poliwrath - evolves from Poliwhirl
Magikarp - Routes 12, 13, 17, 18, 23, Fuchsia City
Gyarados - evolves from Magikarp (Fuchsia City-yellow Only)
Goldeen - routes 4, 12, 13, 17, 18, 19, 20, 21, 24, 25, Fuchsia City, Ceruleon Gym, Cinibar Island, Seafoam Island
Seaking - Secret Dungeon, Route 23, Fuchsia City
Horsea - Routes 19, 20, 21, Cinibar Island, Seafoam Island
Seadra - Secret Dungeon, Route 23
Dratini - Safari Zone
Dragoniare - safari Zone-Yellow Only
Dragonite - Evolve from Dragonaire
Tentacool - Pallet Town, Viridian City, Route 12, 13, 17, 1, Cinnabar Island, Seafoam Island
Psyduck - Route 4, 24, 25, Safari Zone
These aren't all the places but its a guide to help you
If you win against him and then head for the indigo plateux and beat Gary there it will evolve into a Jolteon.
If you beat Gary bu miss him out West of Viridian then he will evolve into Flareon.