Thank you, +Nobilis Reed for this one.
You're exploring ruins for a portal. If you find it, you don't intend to come back. You have horses - and you can't take them through. What do you do?
If you tie the horses up, they'll starve.
One common trope is to tie the horse up loosely so it will break free if you don't come back. Unfortunately, this is actually pretty dangerous - the horse will be left with a rope dragging from its head and they can easily break their neck by stepping on it.
The answer? You just turn the animal loose. Most horses won't leave their source of grain and treats to wander off into the wilds for quite some time, especially if they have a good relationship with their handler. I've turned horses free into the wilderness and gone to sleep and they've been there the next day begging for breakfast. Although horses are nomadic creatures, the likely outcome is that they'll hang around the outside of the ruins for quite some time - long enough for the search - before wandering off.
...like these guys. Are they going anywhere? Nope. Good grass and they know where the rest of the "herd" is.