Children's fantasy from the '60's-mid 70's
I am trying to identify and locate a book that I loved as a fourth/fifth grader in the mid-seventies. It was a book about a girl who heard music coming from the woods at night. One night she follows the sound to find a boy training ponies in an open field. They become friends, but the ponies start vanishing. An evil witch is found to be capturing the children and ponies and turning them into her minions as pigs. The witch yells "Ayeeeeee" a lot. I can't remember the title, author or publisher, but would like to find the book again if possible. Any ideas?
