Tim, what technique did you use to 1) attach the bristles to the brush and 2) what are the bristles composed of? Any one else can chime inon what they use and how they attach it too.
Second question first, they are horse tail.
Drill a 1/4" hole about 3/4 to 1" deep. Take a hank of hair about half the size of the hole, tie a thread around it in the middle, fold it in half and push it into the hole and trim it. I usually start with a thicker piece and remove hair to make it fit. You can use a small stick with a flat end to push the hair in. It may be easier to just break the stick off and leave it in the hole, the hair will cover it. Or you can just glue the hair in but after some use the hair will wear, by pushing it in it is easier to pull it out and replace it. Or you can just glue it in. It is easier to drill the hole in a larger piece of antler and work it down to size than shaping the antler first and then drilling the hole..
Tim C.