Very Common (Everyday):
Pidgey
Spearow
Zulbat
Golbat
Staryu
Weedle
Ratatta

Common (Few times a week to almost everyday):
Nidoran (Male & Female)
Meowth
Poliwag
Cleafairy
Bellsprout
Ekans
Machop
Horsea

Occasionally:
Mankey
Cubone
Diglett
Squirtle
Jigglypuff
Abra
Koffing
Ghastly
Growlithe
Sandshrew
Grimer
Geodude
Eevee

Rare (Caught once):
Ponyta
Charmander
Aerodactyl
Vulpix
Pikachu