A Decoy is an Assist item that can be equipped by Federation Marines in Metroid Prime: Federation Force. When used, a scarecrow-like model of Samus Aran's upper torso is deployed to distract enemies nearby, such as Pirate Troopers and Ice Titans. For every Pirate distracted, the Force is awarded a bonus of 50 points. A short musical cue is heard when it activates. The Decoy wobbles from side to side when placed on the ground, and begins to shoot at enemies until it is destroyed, or it will explode on its own after a certain amount of time has passed. MODs can give the Decoy ice and shock properties, enhance its strength or damage enemies when it explodes. A demonstration of the Decoy can be viewed here: 
It is interesting that Decoys will distract Ice Titans, which unlike the Space Pirates have not encountered Samus before. However, given that they do not appear to have a higher thought process like the Pirates, it is more likely they cannot tell the difference between the Decoy and the Federation Force.
Samus herself appears in the game as a non-playable character.
"A deployable model of the Space Pirates' most hated foe."