My horses best mate had to be put down last weekend.
The carcass was removed on Monday and even though it was covered, my horse knew that it was Billy lying there. My horse hasn't touched any food since and seems very depressed one minute and paces along the fence the next. What can I do to help him through the mourning?

This is a very interesting but also difficult subject, because we can't think like horses. All we can do is ASSUME by putting ourselves "onto their shoes" based upon our knowledge of equine behaviour and intelligence and following our own "natural emphasy" -

I believe horses have some understanding of "death" - the fear of getting killed. It is in their make-up, as we all know. They are prey animals and got hunted to be eaten. This instinct - or "will to survive" makes them "jumpy". They shy, they run when threatened and if they can not escape, they fight.
Horses were not meant by nature to die of old age. The weak and ill got killed first and made it possible for the others to survive. But this did not make the hunted to be what we would see as marthyrs! ( ) - the otheres just got away because they were faster, stronger or in a better position. They eventually stopped running and continued "as usual". I don't think the herd was in "mourning" when one of its weaker members fell victim of a predator.
However, this does not mean that another herdmember might not miss his "mate". We know that some horses form real bonds with otheres. They can be seen all the time grazing together or grooming eachother. I am pretty sure that such a "friend" must feel lost and perhaps search for his companion, despite being a member of a herd. This might be expressed in restlessness, whinnying and perhaps even in what we could interprete as depression.
I understand Billy was your horse's only companion?
Now obviously, your horse has no "back-up". He is alone (which is one of the most frightening things for an animal that was meant by nature to be part of a herd.... and "two makes a herd"!). Your horse is aware of that: His friend is not there anymore and he somehow understands that he has to rely on himself for contentment and survival. His whole world (and equine interaction) has been altered and YOU can only be a small substitute. His surroundings haven't changed and he knows the routine of the world around him but now he must pay attention to all of it all the time as he hasn't got his companion to share his equine day with.
Give him a new friend of his own kind. No matter how much love you give him, you will never be able to replace equine-specific interaction.
I am not a behaviour expert - neither human or animal but I am sure you would enjoy giving a perhaps less fortunate equine a second chance on life as a new friend for your horse and replacement for Billy ?!
If you are in Oz, why not contact "Project Hope" or have a look at the market and save someone?
Happy New Year to you and horse(s)!

My son's pony and my horse were kept together and were almost inseperable. My son's pony died just before xmas however it was an awful death. He was fitting for about an hour before we could finally get the vet to come and put him to sleep. When my son's horse was fitting my horse started barging the stable door and neighing out, we couldn't get him away from the door, he was jumpy and edgy like he new what was happening. When the vet finally came to put Ed(my son's pony) to sleep Major(my horse) hung his head like all the life had gone out of him. For weeks he would not leave Ed's stable and he wouldn't eat. When I think of it we had to lock him up to remove Ed's body. I spent alot of time with major, taking him out trail riding with friends and just sitting with him in the paddock, having some sort of company picked him up and he started eating again. I really believe horses sense death and certainly morn, how else do you explain this behavour. It's never easy loosing a horse and I wish you all the best, hope this has helped in some way.