My theory: The titans aren't actually the fear of the dark, they're the fear of what could happen if the roaring starts. The prophecy is self-fulfilling in this way.
Darkners, when without a purpose, turn to stone. The dark fountain the Knight created turned to stone because dark fountains don't have a purpose when they're already inside another dark fountain. It's like trying to start a new game of pretend when you're already in one. The titan hatched as a result of the fear of this fountain.
What I think it means is that darkners can be created from concepts rather than objects. And since fear is one of our stronger emotions, the beings created from fear are very, very powerful. The Knight could also be the result of a concept given form.