They don't exist."
Luna Lovegood is a most unusual young woman in many ways. One of those ways is her belief in a variety of creatures for which there is apparently no objective evidence.
Luna spotted pale blobs floating in the huge tank in the Department of Mysteries and incorrectly identified them as Aquavirius Maggots. They were in fact brains (OP34).
aqua L. "water" + virius possibly refers to L. virus "venom, slime" or to L. vis "power"
maggot Eng. larval form of an insect, usually living in decaying organic matter
The -ius ending is rather unusual for Latin; most nouns with similar declensions end in -ibus or -us.
Unknown species of creature that many people don't believe exists (OP13)
humdinger Eng. an outstanding thing
Elusive, non-flying creature that lives in Sweden, which seems to be very popular among readers of The Quibbler. Luna and her father planned to take a trip to Sweden during the summer of 1996 [Y16] to search for the Snorkack (OP13, OP38).
Saying that an animal is crumple-horned means that its horns curl in spirals.
snork Eng. a young pig (NSOED).
A spirit of fire; huge flaming creature that gallops across the ground burning everything in its path. Luna believed during Harry's fifth year that then-Minister for Magic Cornelius Fudge had an army of Heliopaths at his command (OP17).
Helios G. the sun + path G. feeling
Someone interviewed by The Quibbler claimed to have brought back a bag of moon frogs to prove his claim that he had flown to the moon on a Cleansweep 6 (OP10).
Known to infest mistletoe quite often (OP21).
Something or other that Luna believed Cornelius Fudge Fudge had during Harry's fifth year. We're assuming it's a creature of some kind because of the context, but we're not absolutely sure... (OP18).
Invisible thing that floats in though a victim's ears and makes his or her brain go fuzzy (HBP7).
wrack Eng., chiefly Scottish The roots of weeds, especially loosened from the soil (NSOED)