I don't know… Kodi is pretty clear about when it's too cold for him. He just sits down and says "no"!
I have a friend who lives in northern VT, and she was out hiking with her dogs at 6F yesterday, though it was sunny and windless there. The other difference is that her dog is a 50 lb-ish mixed breed. The picture she posted did show him wearing a coat and boots, though.
I think it REALLY depends on the breed… certainly Huskies and Malamutes REVEL in this weather!!! And it depends on the clothes we put on them. (and that they will tolerate!) I think if I got Kodi boots (and he'd wear them) that he'd enjoy walks in colder weather… it really seems that his feet getting cold are the limiting factor. Because he's in long coat, wetness is also a factor. I can't keep him dry if it's just above freezing, so I think that tmp, especially if it's also windy, can almost be worse as far as keeping him warm. Wet hair has NO insulating ability!!!
I know that we do not exercise the horses heavily when it's in the single digits… too hard on their lungs. We might walk them, either under saddle with a "quarter sheet" (which overs their back, sides, and hind end) or hand walk them in their blankets, but no heavy work that gets them breathing hard.
And, honestly, _I_ don't want to be out there for any length of time when it's really cold.