Is the fibrous material in this plaster coving asbestos?
If it's traditional plaster, it's horse hair.  When horses were common transport, horsehair was cheap.  Like dogs, horses shed each year.  Stuff was used for cushions too.  More recently they are more likely to use fiberglass, or some form of plastic fiber.

Plant fibers could also be used.  Jute, sisal, flax (linen).