Shiny Pokémon have 25% more HP compared to other variants.
Dark pokemon have 25% more attack power compared to other variants
Metallic pokemon have 25% more defence than compared to other variants
Mystic pokemon have ability to scare pokemon (not always) this helps pokemon to escape from an attack
Shadow pokemon have high immune status
Normal pokemon have no special status
It's true you would get treecko in wild overgrowth during day and night and squirtle both in day and night in shore's end
I got many treecko(but caught only one) in wild overgrowth
sometimes it happens for some pokemon like I was searching for squirtle for a long time still I did not get but I saw my friend getting squirtle in 5min but he did not get treecko.
So keep searching you will get soon
All the best
Make sure you should have also completed elite 4 and champions along with all battle frontiers
If you have already finished it depends on your luck and chances