By nature, snorunt are nomadic. Typically, they travel in packs, but this doesn’t mean that coming across a single wandering snorunt is unusual. In fact, in Hoennian folklore, it’s said that being visited by a single snorunt will bless a household with prosperity, indicating that this was not a strange occurrence to Hoennian natives (although, yes, packs of snorunt are far more common).
As for why they wander, it tends to be for a number of reasons. Some snorunt clans actually claim vast amounts of territory—sometimes even entire mountains—which means that the wandering behavior may simply be that a snorunt is traveling from one nest site to another within a sizable claim. Alternatively, a snorunt that’s completely nomadic (in that it hasn’t claimed a territory at all) may be hunting for a suitable nest site before it begins its search for a mate, or perhaps a snorunt is looking for a new food source. Very, very occasionally, a snorunt may be attracted to the food stores and warmth of a human home, which is very likely why the ancient Hoennians noticed snorunt arriving at their doorsteps. The latter may be the case with your new visitor, although if it leaves again, it may also be that it simply stopped to rest for a bit before pursuing a much larger goal (such as locating a suitable place to build a nest).
As for whether or not it’s okay to treat it, while normally, it’s a bad idea to feed and offer water and a safe home to a wild pokémon, as snorunt stays tend to be brief (assuming the intent for their journeys is something other than to find food or water), it’s fine to offer this one temporary shelter, especially if the weather outside is less than ideal. Additionally, so long as your houndoom knows not to attack or use its fire abilities so close to snorunt, allowing the two to play is perfectly fine as well.
Enjoy your visitor, anonymous!
The Pokédex According to Bill 