The Frog Prince, is a fairy tale, best known through the Brothers Grimm's written version. In the tale, a spoiled princess reluctantly befriends a frog (possibly meeting him after dropping a gold ball into his pond), who magically transforms into a handsome prince. Although in modern versions the transformation is invariably triggered by the princess kissing the frog, in the original Grimm version of the story, the frog's spell was broken when the princess threw it against a wall in disgust. In other early versions it was sufficient for the frog to spend the night on the princess's pillow.
This is a kind of man that I have known. It is someone who seems unremarkable at first but who turns out to be wonderful. Sort of like a "diamond in the rough". Sometimes this can happen in reverse. You can kiss a prince and then after a while he turns into a frog. Has happened to me several times.