I have made brushes out of horse hair. (Learned this from an artist.) First, find a horse. You're looking for coarse hair, so find a workhorse with a mane that stands up. Your on your own negotiating with the horse about snipping off some hair. :grin: Take a hank and twist some paperclip wire around the middle. Fold the hair in half at the wire and stuff it into your ferrule. (Ferrule is any kind of tube; brass, tin, whatever. Like the ferrules on a fishing rod.)
Glue the hair in there with just about any good glue. After it sets, trim the brush with a scissors. There you go; a basic brush.
(You could also use boar bristle, but horses are usually friendlier.)