Horklumps are garden pests that resemble a pinkish mushroom covered with bristles, but it is actually a carnivorous animal rather than a plant.
Horklumps eat earthworms and are the favourite food of gnomes (FB). They have sinewy tentacles and will quickly overrun an average garden.
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.
From the Web
"Horklump" on the Harry Potter Wiki