I’ve encountered a Black Dog twice recently, once he appeared to me in a vision at a Faery Fort, and after I stood my ground led me to meet the Faeries. The second was an actual physical Black Dog, that appeared at Ringlestown Rath when we visited it recently. I’ve been doing some research on the Black Dog in Celtic mythology and couldn’t come across much, except that he could be seen as a negative entity, and was often the Pooka in disguise. I prefer to think of the Black Dog as neutral. Most literature on the subject refers to the British version of the Black Dog, who is seen as negative. Vincent has written his version of the Black Dog of Ireland, as he heard it from his father in his childhood. You can read his story here.