Base Spawn Rates
Outside of factors such as Climate, Area, and Time of Day, each Pokemon Type has a given Spawn Rate, a Base Spawn Rate if you may. This percentage shows what type of Pokemon will appear no matter the cause; however, they can be increased or decreased dependent upon the given factors already stated. (Flying Types and Steel Types are not included in this study, as they are based on their Dual Typing Spawn Rates. Legendary Pokemon are also not included in this, as they are only released via Raid Battles.) Since the Weather System Update, the Weather found in an area now also plays a role in this.
base spawn rates By Type:
normal types - 30%
bug types - 13%
poison types - 11%
water types - 10%
fire types - 7%
ground types - 6%
grass types - 5%
fighting types - 4%
rock types - 4%
fairy types - 2%
electric types - 2%
psychic types - 2%
dragon types - 1%
ice types - 1%
dark types - 1%
ghost types - 1%