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.
Is the black dog legend real? There are a number of legends about black dogs in different cultures. But one can safely say that neither black dogs nor black cats have any supernatural powers, nor are they â¦controlled by evil forces, and seeing, hearing, or having one cross your path does not bring on bad luck.