Flobberworm – A 10-inch toothless brown worm.
So how do they differ from regular worms? what makes them magical?
There’s nothing particularly magical mentioned about Flobberworms, unless looking the same at both ends counts (though regular worms also don’t have a clearly distinguishable head end). They may have a magical property not mentioned, but there’s no mention of any clearly magical ability they have, such as turning invisible.
M.O.M. Classification: X
The Flobberworm lives in damp ditches. A thick brown worm reaching up to ten inches in length, the Flobberworm moves very little. One end is indistinguishable from the other, both producing the mucus from which its name is derived and which is sometimes used to thicken potions. The Flobberworm’s preferred food is lettuce, though it will eat almost any vegetation.”
- Fantastic Beasts and Where to Find Them (textbook)
The Ministry of Magic classifies them as the lowest danger level, “boring”, a classification they share only with Horklumps. The only thing that’s mentioned about them that’s “magic” is that their mucus can be used in potions to thicken them.
“And he wanted you to know you’ll be sorting out rotten Flobberworms from good ones, to use in Potions, and – and he says there’s no need to bring protective gloves.”
- Harry Potter and the Half-Blood Prince, Chapter 11 (Hermione’s Helping Hand)
Why this classifies them as magical creatures is unclear - their mucus doesn’t seem particularly magical either. It’s used to thicken potions, and doesn’t seem to have any “magic” effects like levitation caused by Billywig stings. Non-magical creatures that are known to Muggles are also sometimes used in potions, so that in itself doesn’t usually qualify a creature to be considered magical. There likely is a reason for the Flobberworm being classified as a magical creature - but what it is, isn’t clear.
Flobberworms are shown as possibly having another magical trait in Wonderbook: Book of Potions - they are shown at times to glow.
This may be showing that another magical trait of Flobberworms is that they can glow. There’s also a chance it’s just a visual indication of what to do in the game, but it seems worth mentioning.