Anything you find in grass, caves, or surfing in RBY has exactly a 0% chance of becoming shiny when you trade it forward, due to constraints on the RNG state that are required in order to get a wild encounter in the first place. Static encounters (like Snorlax or Mewtwo), straight-up gifts (Lapras or Eevee), in-game trades with NPCs, and fishing encounters don't place those constraints, so those could be generated with the combinations of stats that would become shiny, but at the original 1/8192 odds, and you wouldn't know that they were set to become shiny without careful reading of stats or actually committing to the trade forward.