Malgyorgs are enemies in Spirit Tracks.
These are sand variations of the Gyorg that appears in The Wind Waker and Phantom Hourglass. When Link is traveling on train through the Sand Realm, he will see these creatures swimming through sand. Usually come in groups of three, they will leap out of the sand and try to attack Link's train.
Simply aim a single Cannon shot at a Malgyorg to defeat it. Link can blow the train's whistle and this will cause all nearby Malgyorgs to jump in the air, making it easier to defeat them.