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.
African wild dog (Lycaon pictus) is one of the large-sized dogs that largely inhabits in Africa and savanna or other wooded areas. These animals are also known as African hunting dog, painted dog, spotted dog, and ornate wolf. The weight of the wild dogs is about 18 â 36 kg (40 â 79 lb).