A Lerdethak is a creature native to the Hork-Bajir homeworld that was one of several monsters created by the Arn to keep any wandering Hork-Bajir away. It looks like a huge gnarled old tree with a drooping blue-rimmed mouth and is surrounded by thousands of vine-like tentacles. It can store prey in a "pouch" in its throat, and its tentacles can keep it well defended from attacks as well as use them like bullwhips. A Lerdethak was controlled and used by Dak Hamee and Aldrea-Iskillion-Falan against the Yeerks in 1968, while Visser Three morphed a Lerdethak in The Forgotten.
- "There was a monster called a Lerdethak, a bizarre tangle of living vines surrounding a ravening mouth."