The Sleestak are a species of large green-hued sapients notable for a combination of both reptilian and insectoid features. According to the Library of Skulls, their population is quite minor, with only 7,000 existing.
Land of the Lost (1974 TV series)
The Sleestaks are regular antagonists in the original series. In the first two seasons, they often attacked the Marshalls when they got into Sleestak territory. In the third season, they become even more villainous after gaining a Sleestak Leader since they actively seeking ways to destroy the Marshalls.
Land of the Lost (1991 TV series)
In the 1991 TV series, the only Sleestaks seen are a group of criminals under the leadership of Shung. In this series, they have an appearance closer to that of the native dinosaurs than those in the 1974 TV series and 2009 movie.
Land of the Lost (2009 movie)
In the film, it's revealed that Sleestaks can reproduce rapidly and that they possess a secondary set of jaws that are only visible when attacking (contrary to the 1974 TV series where the Sleestaks are toothless).
In the past, the Sleestaks condemned Enik the Altrusian to be imprisoned for 10,000 years for his genocidal crimes. However, he manages to escape and murders The Zarn, the current ruler of the Sleestaks. He uses Tachyon Crystals to control the Sleestaks and plans to send his army across time and space to invade and conquer the World.
Long after, the paleontologist Rick Marshall and his friends Holly and Will arrive in The Land of the Lost and Enik sends them telepathic message to tell them to come see him in the Central Pylon. However, after coming they are attacked by the Sleestaks and are forced to took refuge into the Pylon. The following day, Holly is captured by non-controlled Sleestaks for assisting Enik, but Rick and Will save her from being executed and kill the Sleestaks. Enik later arrives with his army of mind-controlled Sleestaks and sends Grumpy to attack them, but the dinosaur finally befriends Rick and defeats the Sleestak army.
After Enik's fail, Will mentions that he has lost control over the Sleestaks and has been attacked by them.
At the end of the film, an egg that was brought by Rick on Earth is seen hatch, and it turns out to be a baby Sleestak.