I exited without saving so don't have a save file I can supply, but it happened to me too.
Again, it was a mare - one that I'd bought as a rescue as a foal, then had age up to mare (playing on long lifespan). 3 other horses on the lot were unaffected (2 stallions, and another rescue mare). Neither mare was pregnant or nursing at the time. And there were 5 feeders on the lot, 4 filled with regular hay and one with prairie grass, plus lots of prairie grass for grazing. The lot was Red Roan Field in Chestnut Ridge, so there's also prairie grass in the field to the side of it.
It didn't happen again when I played through to the same date, but as that was about a week of Sim time a lot of things would have happened differently second time around.
Male horse, had an active ranch hand, 2 full feed troughs, wild hay, no warning message. I didn't know where he was or what he was doing at the time because I was busy fighting the AI to care for my toddler and baby. I do have mods on it, Carls mods as the game is unplayable without them (for example I usually have to reset one or more sims if both my littles have needs at the same time or they make an interruption storm). Hey, maybe he can fix this horse issue that is still haunting the game. =\ Seeming as all the things I have to use the mod for, to make the game playable, still are present. I have the save file separately but it looks like i cant attach it to the post.
Edit: for those still having this happen. I tried something and it worked. It's not a full restore but, I saved the messed up file with "save as" and renamed it to not overwrite the original. Then I opened the most recent autosave (minus 4.5 hours) I saved the family as it was then to the gallery. Then I opened the newest file with the missing horse. I placed the copy of the family in another lot. Then I picked merge house holds. I merged over the missing horse and then removed the other copied sims after. My stallion was still reset 4.5 game play hours but at least it was not the whole household. Not a perfect fix but better than no horse and better than everyone reset 4.5 hours of playtime. =.= If they have an autosave function I don't see why it is reserved to travel only. and not a set time frame...
