It's common wisdom that animals don't understand death and don't experience grief.
With horses, at least, that's absolutely...incorrect. Obviously, we don't know if horses "understand" death - we can't ask them. However, after the death of a stablemate, horses often demonstrate behaviors that are not dissimilar to human grief reactions. Depression and loss of appetite are common. (In fact, it's not unknown for a horse that loses a very close friend to pine to the point where they become sick or even die). Depression and loss of interest in activity can last for an extended period of time.
I've even witnessed a horse perk up, then glance in the empty stall next to her and wilt again, just as if she was turning around to say something to the occupant of that stall and then remembering.
However, they don't show similar reactions if a horse is simply removed. They may show signs of missing them, but not of grief. I have definitely seen more extreme misery expressed by horses when the deceased animal died on the property. So, maybe they understand something.