What made you want to look up xenial? Please tell us where you read or heard it (including the quote, if possible).
Black dog is an oft-used phrase to mean depression. Here's a link that attributes the phrase to Winston Churchill, but I suspect he didn't invent it. I like the way they described it though: Black Dog was Churchill's name for his depression, and as is true with all metaphors, it speaks volumes.
Celtic Black Horse Meaning. The Celts believed the Black Horse was a symbol meaning death and dark forces. It represented the strength of maturity to handle what life brings. The Black Horse was a messenger of esoteric knowledge. He was a keeper of secrets and mysteries.