"Here ...soak your hand in that, it's a solution of strained and pickled Murtlap tentacles, it should help."
-- Hermione Granger (OP15)
A seashore-dwelling rodent native to Britain, this ratlike creature has on its back a growth resembling a sea anemone, which when pickled can be used to promote resistance to curses. The murtlap will attack anyone who steps on it, although it usually eats crustaceans, not people’s feet (FB).
After Harry left his detention with Umbridge with cuts on the back of his hand, Hermione recommended Murtlap essence, which helped the pain immensely (OP15). Harry later recommends the same treatment to Lee Jordan, who in turn suggested it to the Weasley twins when they were seeking a solution to the problem of the boils caused by their prototype Fever Fudge (OP18, OP26).
Hermione makes a solution of strained and pickled Murtlap tentacles for Harry who placed his left hand in the bowl and it helps relieve the pain from cutting it open repeatedly in the detention with Umbridge. (OP15)
Harry sees Lee Jordan (after his detention with Unbridge for telling her that Exploding Snap had nothing to do with her subject, Defense Against the Dark Arts) with the back of his hand bleeding rather badly and recommends the Murtlap. (OP25)
Fred says that the boils caused by the prototype Fever Fudge were sorted out by a couple of drops of Murtlap essence and the Skiving Snackboxes were on a roll. (OS26)