A Lerdethak is a creature from 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 it's throat, and it's tentacles can keep it well defended from attacks as well as use them like bullwhips. Visser Three morphs a Lerdethak in Book #11: The Forgotten.