Ok, this is the age old question. I've been debating with a few mates in the pub and out and about for sometime. I know it's the stupidest thing ever, but boredom does crazy things to you. I think they do... it's pretty obvious... but many disagree 
With crying, I take it you mean crying due to some emotional experience rather than the physiological process of releasing some tears?
Sorry to disappoint you, but for this to happen, you need a fairly highly developed ethogram that also includes self-awareness and the ability to put yourself in somebody else's shoes so that you can experience compassion and sadness about the loss of somebody close (or close to your mind). Horses are not the sharpest knives in the drawer (sorry Hanwombat

) and it would be disastrous for them if some conspecific falls prey to wolves or a cougar to then dwell on it and get all emotional and sad...
You'd have to go all the way up to apes (possibly some monkey species) to find the phenomenon of crying. In that particular context it is usually referred to as distress vocalization (such as separation anxiety in the weaning process which can be especially traumatic for young primates given that their infant care process is drawn out so long), since the shedding of tears in such a situation appears to be uniquely human.