Puzzle for Princess Ellie

Princess Ellie is pony-mad! She'd much rather ride one of her four fabulous ponies, or even muck out the stables, than take tea with the Prime Minister or entertain dull royal visitors. One day, when Ellie is riding through the woods, her brave Welsh pony suddenly whirls around and refuses to go any further. Has Rainbow seen a ghost? It's a real puzzle and Princess Ellie is determined to solve it.