Not a legendary, but I encountered a shiny Hisuian Voltorb at base odds in Legends Arceus but then it and three other voltorbs self-destructed on me knocking me out and I didn't know you could save scum at the time.
So I began roleplaying as a person obsessed with "finding the black Voltorb" to prove I really saw it.
I completed the species research, grinded out the Shiny Charm and went to every outbreak I could find until I finally got it. I made an outfit for my character that matches its colors you can see in picrel and I'm roleplaying it as being a Syngna Suit like the ones from Prokemon Masters.