If you want to save as much of the tail as possible, DO NOT use any comb or brush. Get some Show Sheen or something like that and spray it on. Use your fingers, starting from the bottom, to slowly separate the hairs working your way up. If you're not interested in saving as much hair as possible, you can try using a comb, or my favorite, an Oster tail brush, but with a knot that size you're likely to break a comb or brush. Let us know how it goes.
PS It might take a few hours or a few days, depending on how much hair you want to save.
PS It might take a few hours or a few days, depending on how much hair you want to save.