The Grool is an ancient, mythical creature that resembles a old sponge with its wrinkly light brown skin, and tiny black eyes. It also round in shape and slightly bigger than a chicken egg. It gains strength for the bad luck of any people around it, mainly its owner. The Grool's skin changed color depending on which mood it's in. If it's excited, it will turn pink, then tomato red, then back to light brown repeatedly. Should it be angry, it turns purple. The Grool only moves on to a new owner if the previous one has died and shouldn't be given away. If the Grool's owner gives it away, the owner will die within one day. The Grool can't be killed by force or any violent mean of any kind. However, should the owner show the Grool enough love for it, it will shrink into a tiny ball and explode.
The Grool is the cousin of a more dangerous creature known as the Lanx.
- In the Portuguese adaptation of It Came From Beneath the Sink!, this creature's name is changed to "Grul".