I never thought about boar hair, but I guess it must be like deer hair? That's hollow inside, so it's both stiff and buoyant - perfect for dry flies, which are meant to sit on or in the surface film of the water to mimic hatching or flying insects.
I have seen feathers for sale on Ebay, but I've never tried it myself, even though we have the peacocks etc. I might have to look into that one....