The Shelter aspect: All a healthy horse really needs is a wind break. More often than not, a good stand of trees can provide this. I have a lovely barn and loafing shed for my horses and you will not believe that more often than not, they are standing in the rain/snow rather than getting out of the weather. All they care about is getting out of the wind.
Food: This is a huge one. The amount of hay/grain that would be needed to sustain horses that are constantly stalled would be HUGE. The food itself would require just as much, if not more storage space than the horses themselves. Let's not forget that luperci do not have any of the mechanized machinery that we use today for this either. The hay would all need to be cut by hand, gathered up (this could be done using the horses) and piled. There would be no bales. Square, round or otherwise. I want to headdesk every time I read someone giving their horses a flake of hay... Even if the luperci were to come across a barn filled with old bales, the strings would be rotted as would the majority of the hay. It would all look like one big pile with no semblance of bales left at all. It took less that 8 years for this to happen to a stack of hay in my parents barn after I stopped showing cattle. Also, the horses would not be interested in eatting it at all. At best, it could be used as bedding, so long as the luperci took the time to clean all the raccoon poop out of it, as it can cause horses to get rather sick in a hurry. As far as winter is concerned, horses will dig through the snow and continue to graze on what foliage that is buried under it. Bovine do not do this however, and would have to be hayed in the winter. The horses still likely need a supplement of some sort throughout the worst of the winter months. Especially in more barren areas.
Water: Packing water to horses would be a pain. It would be much easier to let the horses make their own way to water sources or at the very least, lead all the horses to water once or twice a day. Finding/Making water tight containers to pack water in and even holding water for the horses would also be quite chore. Most metal stock tanks would have rusted out since the fall of mankind and back in the time frame of the Apocalypse, plastic type tanks were not all too common as they are today. In the winter, save for really bad cold spells, horses can actually break through the ice at the edges of ponds without our help. Moving water, even easier so, as ice usually doesn't get as thick where the water is still moving. (Leading a herd of horses to water would be as simple as catching the one or two lead horses of the herd, the rest would follow, as is the nature of herd animals.)
Waste: Simply, why would anyone want to spend all day cleaning out the stalls for a herd of horses when they can simply be grazing and dropping road apples where they will actually do some good and fertilize their grazing grounds? Not to mention that all that would really pile up over time and what are Luperci going to do with all that? Sure it makes good garden fertilizer, but the horses are going to produce more than a pack can use.
Exercise/Hoof Maintenance: Horses get bored if left stalled with nothing to do. They develop bad habits, many of which can be really bad for their health, like cribbing. Plus it would take a lot of time to exercise all the stalled horses. Easier to let them blow of steam running around and playing with themselves and looking for food rather than creating all that work for their care takers. Not to mention that horses being left to naturally roam a territory as large as pack lands unencumbered by confinement the less maintenance their hooves are going to need, because they will keep their hooves warn down. Daily riding can also accomplish this.
Time: If a pack as very many horses at all, I'm just gonna go with 8 or more, with all I listed above that is going to be a lot more work than the 1 or 2 wolves that are typically going to be taking care of the horses could accomplish in day even if they did nothing but mess with the horses. Gathering food alone would take the effort of the whole pack for a couple months. If the horses are stalled, that is. Free roaming, you could just have 1 or 2 'herders' that keep an eye on the horses/livestock and keep them from roaming too far. But in all reality, it wouldn't take long for the horses to realize that the pack lands are a 'safe zone' for them. Not only are they safe from attack from the canines that care for them, but they would also come to realize that the extra food they are provided and don't have to seek out themselves would be worth sticking around for.
When does it make sense to stall a horse?
- When they are sick or injured, obviously, and are going to be needing daily attention.
- Any horse that is ridden daily for prolonged periods of time and otherwise would have time to graze for needed nourishment anyway
- A horse that still be trained and therefore being handled daily.
- Mare expected to foal soon, but even this one could be argued. I have always been told that a Mare is not likely to foal while being observed and have never witnessed it myself.
- Newly acquired horse that has not been acclimated to the herd or pack yet