They were mules. I lived there in 1954-5. My parents were teachers. I was in 2nd grade and loved it.
I’ve always loved Grofe’s Grand Canyon Suite, too. Brilliant.
My wife did the mule ride to the bottom with a night at phantom ranch a few years ago. Loved it and would do it again in a heart beat. She was charged with carrying the mail in her saddle bag too a bonus recollection.