During the Delta Episode, when you encounter Rayquaza, you can KO it, and it revives until you Capture it. If I continuously KO it, will it eventually become a Shiny, or is it pre-set from the start?
Rayquaza is probably shiny locked like most legendry Pokemon in gen 6. However, his stats will reset. For instance he could have a brave nature and 301 Atk one time, and after its returned, bold nature and 234 Atk. All Pokemon locked in a certain place (even trainers Pokemon), reset every time.
Hope I Helped