Well, this is an intriguing development. Yesterday morning I decided to quit shiny hunting for a while and just work on training my Pokemon, so I went to Frost Cavern to horde train my Carbink. The first two hordes were just normal Vanillite, which I took out with Rock Slide. However, in the third horde, the Vanillite in the middle of the group was shiny. I used Smack Down to eliminate the non-shiny ones one at a time, then caught the shiny one. Later, while I was horde training my new shiny Vanillite, I ran into a shiny Foongus. Figures, as soon as I give up trying to find a shiny I suddenly get 2 RE shinies in one day.