Yup, it's real horse hair and feathers that are burnt into the clay surface!

After the pottery surface has been smoothed and bisqued, then later heated again in a kiln to about 1400 degrees.  It is removed from the kiln and the horse hair & feathers  are applied to the exterior of the pot.  The hair and feathers burn into the surface of the clay and the black carbon markings are all that's left after washing.  Ferric Chloride is sprayed onto the pieces that have the amber color.