Meganueras are huge prehistoric dragonflies who are seen in Book 1: Keeper of the Lost Cities.
Grady was giving one of them exercise when Sophie came home from school. When she saw it, she was extremely scared, even though it was a younger one and describes its eyes as huge disco balls. It dived at Sophie, and Sophie then screamed. It is not mentioned in the rest of the books. At a young age, they are around the size of a vulture, though when they are fully grown they are twice as big. To give them exercise, an elf will walk around holding them on a thick cord.