The horse hair was the stuffing, but it always managed to get through the covering fabric and as you say was very uncomfortable on bare legs.
It must have been manes and tails because other body hair is quite soft .
Violin and cello bows are horse hair and my daughter’s roof timbers in her 16c cottage are tied together - cross beams, purlin and rafters, by lengths of horse hair now some 500 years old.Thick lime plaster is between the timbers .
The roof timbers take the weight of thick thatch as seldom are thatched roofs stripped completely ,usually more rushes are placed in top of the old ones.
Viktoria.