Horses don't get depressed, that is a human emotion.
You're entirely wrong on that note, my friend. Horses can
get depressed. If they aren't given turn-out, if they get bored with their work, if they are abused or neglected, and countless other situations can make a horse depressed. They can feel many emotions in fact, and depression is one of them.
As for your situation, Beau Baby, maybe you can spend a day observing them while they are in the pasture together. Especially during feeding time. Make sure to stay where you aren't bothering/disturbing the two of them, so that they will continue what they normally do. It sounds to me that they are just trying to figure out the new pecking order, (even if the new horse has been moved out) and with that comes knock-downs on a horse's ego, which can make them feel misplaced for a short while. Keep in mind that you may see lots of biting, kicking, and charging, amd that it's completely normal. But if it doesn't stop in a month, you may want to consider seperating them if you can. Have you changed his exercise/feeding routine? Is their variation in his life to keep him stimulated? And definitely look to see if he's sick. De-worming is a great idea, and also check teeth. Look into ulcers as well. If you're stumped, ask a vet. There's countless other causes that could be thought over, but it really sounds to me like just a little leadership confusion, and hopefully they will sort it out.