A unicorn would be perfect for that. An episode where Hilda discovers a unicorn in the woods. Because Hilda is so pure of mind it'll trot right up and lay its head in her lap, but then she develops a crush on someone and the unicorn starts avoiding her. Once she figures out why she can't go near it anymore she has to try to make her thoughts innocent again, since she needs something from the unicorn and quickly.
Turns out David and Frida can't approach the unicorn either because they both have a crush. Frida readily admits hers is from a band she likes, but David stubbornly refuses to admit who he has a crush on, as it's implied to be one of the two girls asking.