... Luna believed so many extraordinary things ...
Creatures
Aquavirius Maggot: A species the Ministry of Magic are suspected to be breeding, most probably to be found inside the Department of Mysteries. Likely to reside in a tank of water, and to look much like slimy brains. They are probably dangerous; otherwise the Ministry wouldn't be keeping them a secret, now, would they?
Blibbering Humdinger: The Blibbering Humdinger is a creature that most people don't believe in, although these days their existence is slightly more acknowledged. They may be noisy and somewhat spectacular, although we cannot know for sure.
Crumple-Horned Snorkack: A creature that cannot fly and often cannot be seen, it is rumored to bear some resemblance to a pig, and may have curled horns. Sweden is a good place to search for the Crumple-Horned Snorkack, although not necessarily a good place to find them. The Quibbler has many excellent articles on the Crumple-Horned Snorkack, which proves that there is a lot to say about them, although it's not as interesting as what there is to say about Harry Potter.
Gulping Plimpy: A Plimpy is a very boring and conformist creature that you can read about somewhere else. However, a Gulping Plimpy is a different story. For example, they hate Gurdyroots and humans don't seem to like them. We never said it was a long story.
Heliopath: The ever-fearsome spirit of fire, they are known to burn everything in their path, so it is advised that you don't stand in front of them. Cornelius Fudge, the very unpopular ex-Minister for Magic, was said to have an army of these creatures, but this we cannot confirm. Although, we wouldn't put it past him.
Moon Frog: Best harvested after traveling to the moon on a Cleansweep 6, moon frogs love to reside in a bag. Someone has already claimed to have achieved the above feat, but if you did it too, we'd still be impressed.
Nargle: These tiny creatures infest mistletoe, and therefore make them unpleasant to stand underneath. They're also a great excuse for avoiding kissing someone.
Wrackspurt: Wrackspurts go in people's ears and make their brains go all fuzzy, and they're also invisible, which makes them an especial nuisance. They can be detected by flapping your hands in the air, or by observing the vacant expressions on people in the vicinity's faces. And an invasion of them is definitely grounds for re-sitting an examination.
Plants
Gurdyroot: Looks rather like a rather large green onion, in fact some people say that's what it is, but we know better. After all, when was the last time an onion got rid of a Gulping Plimpy?
Miscellaneous
Spectrespecs: A pair of multicolored spectacles that are helpful if you want to look like a demented owl. Also they may reveal invisible spectres, but don't quote us on that.
Umgubular Slashkilter: Cornelius Fudge could have one of these. That's enough to put anyone off.