The Prison guard is a non-unique but non-randomly-generated monster added in the Convict patch. Prison guards are generated only in the quest, as a challenge to the Convict seeking the Iron Ball of Liberation.
If the bribe is paid, the guard will become peaceful (but not courteous—"Good. Now beat it, scum!") and leave the player alone.
Like the Minetown watch and vault guards, prison guards are coded as members of the Yendorian army, and outfitted as such. The prison guard's target AC is -2, the same as for a Yendorian lieutenant. Like watch captains, prison guards have a 50% chance of being generated with a silver saber instead of a long sword.
Reference: Karl Garrison's Convict Patch