Lanx

The Lanx is an ancient, mythical creature introduced in the thirtieth Goosebumps book, It Came from Beneath the Sink!, and its accompanying television adaptation.

It Came from Beneath the Sink!
The Lanx is a cousin of the Grool. They are both ancient, mythical creatures. Both creatures feeds off their victims' energy, but the Lanx is supposedly far more dangerous than its cousin. The Lanx latches onto someone until it drains all of their energy.

The Lanx is first referenced about halfway through It Came from Beneath the Sink!; the characters find information about the creature in the book Encyclopedia of the Weird. However, the Lanx doesn't appear until the end of the story. After defeating the Grool, Katrina Merton's dog, Killer, finds a Lanx underneath the sink.

The Curse of the Creeping Coffin
Based on the reader's decisions, a Lanx can appear in the eighth Give Yourself Goosebumps book, The Curse of the Creeping Coffin. The reader sees eyes peering out from under their sink, and they must decide whether the eyes belong to a Grool or a Lanx.

If the reader chooses to tell Robin that there is a Lanx under the sink, Robin stops believing the reader and leaves. The reader is then left with the Lanx. It has glowing eyes and can be heard chuckling. The book seemingly punishes the reader for choosing the more dangerous character.

Physical appearance
The Lanx resembles a potato, except it has a mouth and jagged teeth. In the television adaptation of It Came from Beneath the Sink!, the Lanx doesn't appear to have eyes. However, in The Curse of the Creeping Coffin, the Lanx has small, glowing eyes.

Trivia

 * In real life, the term "Lanx" can refer to a genus of snail. Additionally, a "lanx" is a type of ancient Roman platter that was often used for sacrifices.