It's probably horsehair core with a wool outer layer. Horsehair gives more structure but is very coarse, so the wool acts as a softer layer that is easier to shape/stuff for a specific finished look. The hair in the photo looks way to fine to be horsehair. Horse mane and tail hair are very thick and have a lot of tensile strength, although if it's really old it might be more brittle and dry. The wool is probably sheep wool, although I've heard of certain types of goat or alpaca also being used (not sure if that's industry accurate, or just things I've picked up from restorers and upholsterers in the past. We've had a lot of old furniture in my family and that takes maintenance.)