An iron horse with a flaxen tail. The longer the horse runs, the shorter his tail becomes. What is it?

Why this works

This riddle cleverly uses metaphor and imagery to describe a needle and thread. The "iron horse" represents the needle, which is often made of metal (hence "iron"), while the "flaxen tail" refers to the thread, which can be made from flax fibers. As you sew (or as the "horse runs"), the thread is pulled through the fabric, causing it to get shorter, thus creating a playful connection between the action of sewing and the imagery of a horse galloping with a diminishing tail.