Scouts were units that were unlocked with the Free Spirittech before the Myriad update. Scouts have two main abilities: Scout and Sneak. Scout allows the Scout to explore a 5x5 radius instead of 3x3. Sneak makes the scout not affected by movement barriers like Mountains (you still need Climbing).
As of the current update, Scouts have been removed, due to AI management and tech tree issues. Scouts have somewhat returned in the form of battleships, due to them having the scout skill.