Here's a description from a story from Emma Hardinge Britten that I recently found that I do like from 1820 England:
We were about to sit down to tea when Mdlle. Gronin called our attention to the steady light, round, and about the size of a cheese plate, which appeared suddenly on the wall of the little garden directly opposite the door of the hut in which we sat.
Before any of us could rise to examine it, four more lights appeared almost simultaneously, about the same shape, and varying only in size. Surrounding each one was the dim outline of a small human figure, black and grotesque, more like a little image carved out of black shining wood, than anything else I can liken them to. Dorothea kissed her hands to these dreadful little shapes, and Michael bowed with great reverence. As for me and my companions, we were so awe-struck yet amused at these comical shapes, that we could not move or speak until they themselves seemed to flit about in a sort of wavering dance, and then vanish, one by one.
I particular I like the "black and grotesque...carved out of black shining wood" bit the most. The very first image that come to mind when I picture this is the Irish Shillelagh, which are clubs (used as ancient dueling weapons) made by being "smeared with butter and placed up a chimney to cure". This gives them a very pitch-black, shiny, appearance. I own one of these myself.
From the history, it seems Kobolds were found, more often then not, in mines or doing chores around the house like Hobgoblins. One important distinction, however, is that Kobolds appear to be far more kindly creatures than Hobgoblins. In light of this for my campaign in the future, I would most likely make them be fairly neutral in their natural environment, or perhaps slaves to evil overlords. But I would avoid making them out to be evil or vicious as that seems misaligned to their real nature.
A separate wikipedia entry states that, "house kobolds usually live in the hearth area of a house", which could explain the black wood appearance. I imagine them being fey spirits inhabiting animated small wood-like bodies, blackened from the black pitch and soot from hiding in fireplace hearths and filthy, coal covered, mines. A few may have horns, but most would be slightly smaller than Goblins, and slightly bigger than Hobgoblins.