I don't know much about chain driven ATVs but based on my dirt bike knowledge it looks like a chain idler pulley. A lot of times they'll be on a spring tensioned arm under the chain where it returns to the wheel sprocket. Sometimes they are just bolted to the frame and the chain rests on top of it to keep the chain from rubbing on the swing-arm when the suspension is extended.
2010 Polaris X2 550
ATV-Guru Cobra bumpers, Polaris front and rear A-Arm protectors, heated hand and thumb grips, 3500lb winch, hand guards, gun scabbard, 3" passenger foot risers, Glacier III plow, HID Lights, Tiger Tail, 9" Windshield