Horse hair, boar bristle and, yes turkey beards all work well. I do as Tim does and don’t epoxy so I can replace as needed... especially if you’ve a fancy carved handle out of deer antler. I have an old, real old, car wash brush made of horsehair and have enough of the stuff to last generations. Whatever ye choose, the shorter ya cut it, the stiffer she gets, to a point.
That being said I often wondered why they call it a pan brush. T’aint very useful for cleaning a fouled pan; I use a shirttail or rag for that. A brush is useful for whisking unburned prime say when you want to make your gunne safe. I find it also handy for briskly brushing other parts of the lock such as the frizzen spring and bearing area and the cock itself when shooting many shots for the beef.
I suppose in this modern day, some sort of synthetic bristle would work well and many look like a turkey beard. My experience synthetics also wear out, or pull out, over time too. It all depends on what stiffness ya like and what is handy, I reckon.