I had no idea shiny Ho-oh was even possible to encounter. I've only done one Ho-oh raid so far, and it wasn't shiny, so let's see what happens for the next two weeks.
Up
0
Down
Ho-Oh can't be shiny. Not yet, anyways. Being a raid boss doesn't mean that there is a shiny option for it, just those raid bosses that are already shiny (magikarp and sableye) have an increased chance of being shiny when catching from a raid than from in the wild.