"Er--how are the flobberworms?"
-- Ron Weasley (PA11)
The flobberworm is a ten-inch, toothless brown worm which eats vegetation, preferring lettuce. The mucus exuded by a flobberworm is sometimes used to thicken potions (FB).
Malfoy maliciously clues in the trio to Hagrid's sudden absence from lessons by giving them a newspaper article written by Rita Skeeter. In this article Skeeter reveals Hagrid's 'shortcomings' supported by inaccurate quotes from Malfoy about being injured during Hagrid's lessons. "What's this rubbish about him getting a getting a bad bite off a flobberworm? They haven't even got teeth!" is Ron's reaction after reading the article (GF24).
Hagrid, after losing his nerve with the hippogriff debacle, had his third year students raise these for a semester, which was completely pointless as they prefer to be left alone and to do nothing (PA6, FB). They live in damp places such as ditches (FB).
In his retirement, Professor Kettleburn kept a number of Flobberworms at his home in Hogsmeade, and, being physically unable to partake in the Battle of Hogwarts, he did his part by throwing them all at passing Death Eaters through his attic skylight. It was not very effective, but his effort was appreciated (Pm).
The information regarding Professor Kettleburn and the Battle of Hogwarts comes from the new Pottermore ebook.