As much as I love the idea of a dog sled, I can't make it work as a player mount mechanically, because there won't be snow everywhere. I'd be happy to see them as a transport service though, where the means of transport varies with geographic features. Although a player mounted horse may be able to traverse snow (maybe even with a slow cart attached reducing its speed), I doubt that a dilligence based service would be able to ignore snow.
So for services; mammoth, horse dilligence, dog and raindeer sleds, depending on local geographic features making it feel natural. Please add more
Player mounts; horses, wart hog (slightly oversize), (pack) mule (slower, but packs more and ignores terrain/slopes). Ehm, slow today, please add more
Services would have more depth to it, I just included those dealing with animals.