These are the main Types of Pokemon that can be caught in each area and their general ratios. However, there is a small chance that ANY Non-Legendary Pokemon will appear ANYWHERE!
For Mods: Roll a Dice (1-6) for how many will appear, up to 6 at a time.
Roll a 1-1005 Dice (You may do this for each individual pokemon that appeared or group them up to the same type)
Select Type for the Location in the Table Above according to the roll.
Should you roll a 1001-1005 then roll for ANY AND ALL minus legendaries.
Then use
https://randompokemon.com/ or something similar to find out which Pokemon it/they are.
These Pokemon may be together in a group or scattered around the place, the choice is yours.
If appropriate you can roll a dice for whether they are Male or Female. Should the trainer choose to approach/enter combat with one or more of the nearby Pokemon you will roll for the Pokemon's Level. This will be between 1 and Trainer's Highest Pokemon Level +10. This way the level can never exceed 10 higher than the highest Pokemon in the Trainer's Party.