What did mary alice mean when she said, “grandma and i went out on shank’s ponies.” this is in the book "a year down yonder"

In the book A Year Down Yonder, we meet Mary Alice, who is a 15 year old girl who has to move with her grandmother to a rural town in Illinois. Mary Alice is originally from Chicago, and the book shows the differences that exist between these two places.

One of these differences is the use of language. An expression that comes up in the book is that of "shank's pony." This expression refers to walking, or travelling by foot. Shank is the lower leg, between the knee and the ankle, while pony references the use of horses as transportation.