It's like English's "the one". For example if you say "I like the blue book. I also like the red one" In Japanese it would be something like: 「私はその青い本が好きです。そして、その赤いのも好きです。」. The は in your examples is just of course the topic marker. In my example the thing I used の for isn't the topic but if it were, it would have は on it. For your first example, the literal translation would be like "The scary one (or thing)....the scary one....is dragons." Your second example is a bit harder to translate somewhat literally and have it make sense but it's like "In this world the one that can love you, is one person.". It's taking "The one that can love you" and making that the topic of the sentence so that they can state later that there is only one person who matches that description.

btw こわい has only one い.