Agent K

Mesprit and Azelf. 'How could they be Nightmare Fuel?', I hear you ask. Well, they may be cute, but the myths say that they are Nightmare Fuel.

Think about it. If you hurt Mesprit, you lose all emotion and if you touch Azelf, you lose all willpower. (or hurting Azelf makes you lose Willpower and touching Mesprit makes you lose emotion) Now, think what you're doing to lower their HP for a capture. That's right, you're making your Pokémon lose their emotion/willpower just so you can catch a legendary. Nightmare Fuel indeed.