Sandtrout were creatures native to the planet Arrakis. They were described as resembling large freshwater leeches, amorphous blobs or slugs.
They were the premature form of a sandworm and if squeezed, they excrete an extremely potent form of Spice which gives one an incredible energy boost. Also, children in Fremen sietches attached them to sticks and play games with them to entertain themselves.
Sandtrout had the ability to transform a whole terraformed planet into desert by encapsulating large bodies of water allowing the Sandworm to survive, since Sandworms cannot survive in a water-logged environment.
Sandtrouts were also used by Leto Atreides II to turn himself into a giant Sandworm. He managed to live through the metamorphosis because his blood is spice-concentrated and this led the sandtrout into thinking it was water. The first formed a glove over his hand, and the next ones linked to it by their cilia until most of his body was covered.
Later, Sandworms could only sense the sandtrout covering his body and not him, so the Sandworms drew back to ensure they do not eat their own kind. Gradually, they transformed his body until his metabolism and biochemistry were a hybrid of human and Sandworm. However, he also modified the trout in the process, making the resulting sandtrout tougher and more adaptable, allowing them to be settled on other worlds. Thus he ensured the survival of the sandworm species.