A Sentreye is a Luminoth security drone. They appear to be red circles embedded into the rock of Aether. They are, as the name suggests, sentries for the Luminoth. Darkling Sentreyes are called Corrupted Sentreye.
"The Sentreyes were designed by the Luminoth to protect their camps on Dark Aether. The Ing learned how to possess them and turned them against their former masters. Many still remain throughout Dark Aether, now serving the Ing as they served the Luminoth before them. Most weapon types will stun the Sentreyes, charged light energy will destroy them."
Quite obviously, their name is a portmanteau of sentry and eye.
Oddly enough, normal Sentreyes are not in Metroid Prime 2: Echoes. This is believed to be caused by the fact that Sentreyes were primarily (if not exclusively) used on Dark Aether to protect Luminoth camp sites. As such, normal Sentreyes cannot be scanned or found anywhere. All Sentreyes are Corrupted Sentreyes, so there is no known Logbook entry for them. This could also be why they are referred to as "corrupted" instead of "dark". The Sentreye is one of several creatures ingame that do not have a light variation.