Wandering pirates are monsters that appear in dNetHack and the Pirate patch. When carrying the Pirate quest artifact, the Treasury of Proteus, wandering pirates will occasionally appear and fight the player for the Treasury. These monsters are otherwise not randomly generated in the game, although skeletal pirates will randomly appear in the Pirate quest, and several skeletal pirates and damned pirates are guaranteed to appear on certain levels of the quest.
In bones levels, dead Pirates may return as skeletal pirates.
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"Are wandering pirate species part of random monster generation in Slash'EM Extended or SlashTHEM?"
Wandering pirate generation uses a mechanism separate from ordinary monster generation. While the player has the Treasury in open inventory, there will be a 20% chance that whenever a new monster is randomly generated, it will be replaced with one or more wandering pirates. Wandering pirates will always be of a type appropriate for the theme of that part of the dungeon: damned pirates in Gehennom, githyanki pirates in the end game, skeletal pirates elsewhere.
The difficulty, attack strength, and armor quality of the type of wandering pirate the player encounters increase with the stage of the game where they appear, but otherwise they are significantly weaker than the monsters that could have been generated otherwise, especially because of their lack of special abilities or attacks. Although they all implicitly want the Treasury, none of the three types is designated as covetous, so they can't teleport to the Pirate carrying the artifact, and none of the monsters have a thieving attack. They will also fight with other monsters and one another.
By preventing more difficult monsters from being generated, this effect of carrying the Treasury may actually be more helpful than harmful to the player. Also, in dNetHack they provide a way to acquire more bullets for the Pirate's starting flintlock, as anything firearms-related is otherwise incredibly rare in this variant.
Skeletal pirates first appear during the Pirate quest. In addition to making up about 55% of all hostile monsters randomly generated in the quest, several are guaranteed to appear on each level of the quest. They will continue to appear as wandering pirates after the player leaves the quest.
Skeletal pirates are generated with a rusty cutlass (scimitar) or knife and a rotted leather jacket or pair of high boots. In dNetHack they also get some rusty bullets and either a rusty flintlock or another knife.
Damned pirates appear as wandering pirates while the player is traveling through Gehennom with the Treasury. In addition, a few are guaranteed to appear on the lower levels of the quest.
Reference: Chris's Pirate Patch