When the hero of the Child ballad The Lord of Lorn and the False Steward is forced to trade places with an impostor and swear never to reveal the truth to anyone, he tells his story to a horse while he knows that the heroine is eavesdropping.
In the Child ballad "The Lord of Lorn and the False Steward," the hero, after being tricked into swapping places with an impostor, vows secrecy.  He confides in a horse, unaware that the heroine is eavesdropping on his confession.