Maggots are the larval forms of some varieties of fly, thought the term is commonly extended to the larva of insects in general. Maggots are not limited to non-magical insects, a certain species of magical giant grub exists. Luna Lovegood also believed in the brain-like Aquavirius Maggots, though it is likely that these do not truly exist.
If the Patronus Charm is attempted to be cast by a Dark or otherwise not pure-hearted wizard, who would otherwise be magically capable of casting one, rather than the intended effect of a spirit guardian bursting forth, a horde of maggots will be conjured from the caster's wand, which will attempt to consume the caster. The first recorded instance of this happening is with the Dark Wizard Raczidian.