The sentinels were originally programmed soldiers in a mobile Fortress command game. After Megabyte corrupted the fort and the soldiers within to serve him.
After acquiring a replicator, Megabyte was able to expand his army further. When the replicator was destroyed by the new guardians, Megabyte has already acquired a huge number of Sentinels.
The Sentinels have the same appearance as Hack and Slash, Megabytes original henchmen in the original series.