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.
About the African Wild Dog The African wild dog, also called Cape hunting dog or painted dog, typically roams the open plains and sparse woodlands of sub-Saharan Africa. These long-legged canines have only four toes per foot, unlike other dogs, which have five toes on their forefeet. The dog's Latin name means âpainted wolf,â referring to the animal's irregular, mottled coat, which features patches of red, black, brown, white, and yellow fur. Each animal has its own unique coat pattern, and all have big, rounded ears.