This is something I've been wondering for a while. Outside of Wally's wild Ralts and Professor Juniper's Minccino, can enemy or NPC trainers in any other game have a chance to have a Shiny Pokemon?
(There are various Trainer Tower battles in Fire Red/Leaf Green that have Shiny Pokemon, but these are always forced to be Shiny and not a random chance.)
Only in the Battle Frontier / Subway / Tree / etc., where they have the same probability as random shinies. Other than that, there is one trainer in the main series games with a guaranteed shiny: a sightseer (Markus) in Seafolk Village in SM/USUM (credit to Dragonblade). After searching on Bulbapedia I couldn’t find any other examples of trainers with shinies, which would be predetermined for any trainer outside of the battle facilities.