How to add Pokemon that can be found in grass but is uncatchable?

[Anuran]

I'm using Pokemon essential v 17.2
I have a question ---- How can I add Pokemon that can be found on grass in a specific place that is uncatchable

 

[James Davy]

Make an event at that spot that will turn on a switch that prevents Pokémon from being caught, start the encounter and turns the switch back off afterwards. This is similar to programming an encounter with a shiny legendary Pokémon. Its trigger will be player touch. You will need to use a self switch if this encounter is a one-off.

Programming the uncatchable Pokémon script is as follows: in PokeBattle_Battle, def pbThrowPokeBall(idxPokemon,ball,rareness=nil), add the following code below the def line:

if $game_switches[nnn] # change nnn to a switch you're not using
  pbDisplay(_INTL("It dodged the thrown ball! This Pokémon can't be caught!"))
  return
end

This is derived from Desbrina's Lavender Town Ghost tutorial found in the wiki.