I probably don't know any more about Quedlinburg than you do at this point. As for the white horse, it was the symbol of the Duchy of Brunswick Luneburg, the successor to the original stem duchy of Saxony. The white horse on red is also the symbol of Kent, brought there by Hengist & Horsa (Saxons). For sources, I can give you the Atlas Maior by Jean Bleau, a world atlas from 1667 filled with heradry and bits of information about it - you know how those old maps are. Unfortunately, I can't seem to find the right pages online.
I can't remember where I originally picked up the information, though.