The HK Guardian Droid was a model of droid whose superior armor was almost impervious to blaster fire. After realizing this, they became difficult to control and ultimately contributed to the common prejudice against droids. HKs were no longer in service by the early years of the Galactic Empire.
When Lonn Idd's repair station was under attack by the Fromm Gang, the protocol droid C-3PO donned the aged shell of an HK Guardian Droid stored in a spare parts storage room, allowing him to survive a group of hover guards' and destroy the sentry droids.