It was Churchill who called his depression a black dog.
“I don’t like standing near the edge of a platform when an express train is passing through. I like to stand right back and if possible get a pillar between me and the train. I don’t like to stand by the side of a ship and look down into the water. A second’s action would end everything. A few drops of desperation.” – Winston Churchill (1874-1965)
Nick Drake ended his own life so I think he knew all about the black dog.