Picture a fluffy dog, eg., a Golden Retriever. Rear view. Long fluffy, sometimes billowy hair slightly lower and to either side of tail.
I call them his “curtains.”
Not that left-field of a question, because there is a word for the longish fur off the back of a dog’s front “forearms” (I believe only the front, but I’m not sure): “feathers.”
That longish hair on the lower portions of the legs and on the tail, I have always seen referred to as “feather” (a collective noun in singular form). I’ve never seen it used this way in the plural form “feathers”.
The same word is also used for the long shaggy hair on the lower legs of some breeds of horses, but not for the long hair that all horses have on their manes and tails.
I think ‘feather’ or ‘feathers’ is the most common word, but I’ve often referred to them as britches or pantaloons. I once had an Aussie whose legs changed color right where the feathers stopped, so we called them her hot pants.