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.
Description: African wild dogs have long, thin bodies, large oval shaped ears, short broad. muzzles, and long muscular legs. They come in a variety of colors (black, white, tan, yellow, brown, and gray) and patterns. Each dogâs pattern is. unique and bilaterally asymmetrical (each side doesnât match).