Has anyone experienced random Pokemon gaining the Shiny form in the Alola Pokedex even without seeing them?

I've got a Shiny Golduck in my dex but I have DEFINITELY never encountered one. I am always on point with my Shiny encounters. Hell, my first was a Gastrodon which looks exactly the fucking same as the regular one.

I also have entries for Shiny Pikachu (both genders) and Shiny, non-Alolan Raichu (both genders). How the fuck did that happen? Is there a trainer battle against two opposite gender, non-Alolan Raichu that I just forgot? I remember the Exeggcute one, but that's all.