Horse experiencing depression?

My little guy seems really down in the dumps.
He had a pasture mate, of 5 years, Prince, they always got along/looked out for each other. Well, because he was never getting ridden anymore, Prince was sold and Putts (my horse) was put out with 2 new horses who he doesn't get along with, so he wanders around alone.
Obviously he was frantic the first few days without Prince, neighing and just confused like you'd expect, but now, 2 weeks later, he just seems like not himself.
He's not lethargic, he's not sickly, he's just quieter than usual (he's usually pushy, headstrong). He's also spookier than he usually is. All these 2 weeks he's spooked at things he never spooks at (he is by NO means a spooky horse at all)... He honestly seems depressed and scared!
I'm not complaining, I mean I'm liking the way he's listening now and being a quiet, sensitive horse, but its just not like him no be so scared of the world!
I'm sure he'll come around with time, but right now I honestly think he's depressed!
Have you ever experienced this with your horses?

He feels traumatized at the loss of his friend. He should adjust eventually. It is a lot of stress to him now.

Sounds like he used to be the boss with his old friend, and now he's not the boss anymore, resulting in more sedate behaviours....it's amazing how the totem pole can change......goodluck

I think Muppetgirl has probably hit the nail on the head. Something that we all have to remember is that horses, and any other animal for that matter, do not have emotions or feelings, psychological or physiological, anything even remotely close to those of humans. That is not to say that they don’t have emotions or feelings, just that they are an entire world away from what we have. And though this is not my field of research it is one I have a professional interest in and yes I have studied this sort of stuff at post graduate level at university so have some professional authority to be able to say so. There are two things at play. One human, the other to do with the animals physiology. We as humans have evolved the most sophisticated ability to empathise with others of our species of all the other species on earth. Part of it is language, but a huge amount is our feeling and emotions (they are analytically separate) abilities and the somatisiation of these abilities (and furthermore the ability to read these in others to a fairly accurate degree); it is something we are taught from birth and do till the grave. And we have a natural tendency to do it with all other animals too; especially if we love our horses. The trouble is we tend to deploy this process as if we were dealing with another person, or read behaviours from the animal as if they were from a human and hence tend to anthropomorphise animals. On the flip side though is that other animals, even our closest primate cousins, don’t have the physiology to support the same sort of feeling/emotional complexity as humans; it is just a simple fact of biology they have no where near the neurological complexity to support human like thoughts, feelings, emotions or behaviours. They may have some sort of horse version of these things, but it wont be anything like what people have, and the somatisiation of it wont be the same. In other words just because you think your horse looks depressed doesn’t mean he/she is. So I wouldn’t read too much into it, he will probably sort out his place with his new palls and be back to normal.

Yes Andrew....that's exactly what I meant.......

There has been a shift in herd dynamics/pecking order.....therefore a shift in behaviour to adapt to the changes......survival of the fittest, for every action there is a reaction

I agree that it could be due to a change of dynamics. It sounds like he's lower on the totem pole now, it happens. I'm sure they'll figure it out.

My experience, my friend sent her horse for training this summer and then when he got home he was in a pasture by himself for a little while as she was working on getting him out riding alone on trails ( he is herdbound ). When he was reintroduced to the other 5 horses, he went from being strongly at the top with his partner in crime Tyson, to being right smack at the bottom. One day to our surprise he was told off by my Nugget, who has always been the very low man, and he accepted it and walked away, head down. Major change of behavior for this horse, definitely different "personality" before and after but it happens, nothing to worry about.

I realize this. I just always thought animals mourned the loss of their close 'friends'. I remember in like 5th grade learning about the elephants and how they mourn the loss of their buddies, but horses aren't elephants, either.
I know it's pretty silly to think he's actually depressed, but in a way I think it could more be mourning that his friend is gone? I'm trying to get into a horse's mind.. Wouldn't a horse think if one horse left, it would think it died?.. I don't know.
I'm going with a change in herd dynamics, like many said above, because that makes a lot of sense.
Thanks everybody!

He's walking on eggshells now, because he is at the bottom of the pecking order. This is why he's spooky, has to be prepared for whats going to be next.
He is upset and in the dumps, because he lost his buddy. It will take time and he will be back to normal.