Ever have the campaign where you "have" horses? You ride every where on them and once you get there they seem to disappear till needed again. After all horses are not much good in a regular dungeon and once you get somewhere they tend to just be in the way. Its not like a horse is cheap to acquire or anything so a nice DM will feel its rude to just kill it or something while your away. What should you do with your horses?
If your a DM and don't just want to out of hand kill the horses while the players are away without a reason but feel they should do something about it I do have a suggestion. Have them hire a teamster or some similar type of hireling to watch the horses and whatever animals you bring but need to leave outside the dungeon. There should still be a chance of something happening but with a trained person looking after the animals it should be greatly reduced. Now not only do you have a reason to leave the horses alone but you also have a built in money sink. Sure its not that much money but its cheaper then new horses every trip between town and dungeon so the players will be okay with it because it makes perfect sense. It also means you don't end up leaving your poor horses tied up in the wild if the whole party dies or something.
For you Evil DMs out there who want to justify your horse slaughter while the players backs are turned I have some help for you as well. Random wilderness encounters, its that simple. Why should they only happen when the players are around? What bear could resist the chance to chow down on some free food tied to a tree. If the players are in the dungeon long enough roll for a random encounter involving the horses. Not only does this justify your horse killing spree but also gives you interesting results. After all you can describe whats left, its also possible that not all the horses die and having one horse left provides some free flavor. If the horses left alive are hurt then the players will have to do something about it. If the random encounter is something like bandits, even better. Don't have them kill the horses, have the bandits steal them. A player who sees his horse dead and goes "oh well" might respond differently to a stolen horse. Stealing something from a player makes them want to get it back so they might end up tracking the bandits halfway across the world just for revenge. Also you can roll the combat while the players are in the dungeon and make the players nervous about all the dice you are rolling with nothing seeming to happen.
To finish up don't just kill/ignore the horses. No matter what you want to happen to them there is a better way then DM fiat. Whether you want to kill them or forget them have a reason for what you do. Also don't just let the players leave the horses where ever they want with no care to it. If they leave the horse tied up out in the open in the middle of a dangerous area they should come back and find the horses dead.