This garden pest resembles a pinkish mushroom covered with bristles, but is actually a carnivorous animal rather than a plant.
Horklumps eat earthworms and are the favorite food of gnomes (FB).
There is some conflicting information as to the usefulness of the horklump. Newt Scamander says that the creatures have no use, whereas horklump juice can be collected in Pottermore, presumably to use in potions. It is likely that the use of horklump juice was discovered after Scamander's time, and thus is not mentioned in his book.