From Muppet Wiki

The Garthim are large clawed warriors of amazing strength. The Garthim carried out the commands of skekUng the Garthim-Master. The creatures were created by skekTek the scientist and lived in caverns beneath the Castle of the Crystal. The Garthim were dispatched to ravage Thra and returned with captive Gelfings or Podlings for the Skeksis to enslave and drain their essence.

Physically, they resemble a cross between a beetle and a crab and don't appear to be 'alive' in the purest sense of the word: at the end of the Dark Crystal, they simply fall apart like suits of armor, revealing nothing within. It's possible they may be a form of robot.