First, I bring them inside. I soothe them. I make them feel loved. I Pinkie promise never to abandon them and to always love them with all my heart and soul. Then I send this message out to whoever left them: When I find you, the things I'll do to them will make Hell seem like a pleasant alternative, you disgusting scumbag.
*hears a knock* ? *opens door and looks down* Oh Dear celestia! *puts applejack in basket, picks it up, and brings it inside* don't worry... i'll take care of you *few years later they leave to ponyville and i continue to search for the mare/colt who left them there*