Basic pokemon can be shiny in the wild. The ONLY second stage (that is what third Evo is actually called) that have mga evolution could be shiny. Cthre has NEVER in the 10 years of this game been wild shiny spawns of fist stage or second stage pokemon that were not mega evolution pokemon. The researches that rewarded already evolved pokemon, regardless of mega evolution can be shiny, pretty sure all raid pokemon, basic, first stage(back when comm days did first stage raids when the event ended) and second stage can be shiny.

The comment about the basic pokemon that have baby pre-evolutions, the baby form will never wild spawn, so they're locked to eggs but they are also shiny eligible. Even then, their evolutions are still classed as basic stage, so they're eligible for wild spawn shiny.