Ok so basically today I got Pokemon Ultra Moon for Christmas and while I was leaving the cave for the first trial challenge, I encountered (And Captured) a shiny Diglett, and I wasn't even trying to find a shiny Pokemon! Another thing was that I captured it on my first turn in the encounter with a Great Ball! It is female and Lv. 10 BTW.
NOTE: I am not the best drawer in the world, so expect errors and mistakes blah blah blah, so yeah.