I have a 'black dog'
The 'black dog' metaphor is one of the ways people use to help them understand their depression better. 'Black dog' is the one who wakes us up in the middle of the night with worrying thoughts, 'barks' at the slightest noise, 'chews' up our memory and concentration, and 'chases' away our confidence and self-esteem. Sometimes he can take over our entire lives, so we become the 'black dog'.
Using 'black dog' as a metaphor can also help increase our understanding of what can help. For instance the two things known to have a significant benefit in depression are,
Exercise. Like any dog, 'black dog' is happier and calmer if he is regularly exercised.
Sleep. Making sure you get enough sleep can help you cope better with the days when 'black dog' is trying to 'chew' up some of your most precious things, such as your energy and enthusiasm for living.
If depression is like a dog, then perhaps you can teach this dog some new tricks. Perhaps he can be trained, put on a lead, or made to come to 'heel'. Perhaps when he 'barks', you come to understand that just like a real dog, he is trying to warn you of something, which you need to pay attention to. Or perhaps, when you discover what he is 'barking' at, you realise this is something you don't need to worry about.
Maybe even using 'black dog' as a metaphor might enable you to develop self-compassion, which is known to help with depression. If your depression is like a dog, then perhaps you can develop compassion for this part of yourself in a similar way to having compassion for a companion animal.
Using your own way of understanding yourself through metaphor, can also be used to enrich your therapy with your practitioner. Metaphor lends itself really well to creative therapeutic approaches, such as art, storytelling, and roleplay. It opens up new avenues of exploration, enabling you to look at what those 'tricks' could be, or what this thing is your 'black dog' wants you to pay attention to.