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.
WordReference Random House Unabridged Dictionary of American English Â© 2016. suâ¢riâ¢cate (sÅÅrâ²i kÄtâ²), n. Mammalsa small, burrowing South African carnivore, Suricata suricatta, of a grayish color with dark bands across the back, related to the mongooses and having social behavior similar to that of prairie dogs.