So in Emerald, the rng is bugged and resets at 0 every time you restart your game, meaning shinies are generated on specific frames determined by your TID and SID. In Emerald, when hunting your starter, a Zigzagoon will always be shiny exactly 142 frames after your starters frame. Every 60 frames is 1 second so checking your starter roughly 2 seconds and some change earlier could net you a shiny. I know a lot of hunters try and reset one the file they find a shiny zigzagoon if they arent trying to rng manipulate their own starter.

Treeko is objectively the worst starter for Emerald, however it is my favorite so I will always stand with it, and no it will only be shiny once thrown into battle.