The Unused Gang 9 is, as the name implies, an unused gang appearing in the game's code in GTA San Andreas and GTA III. When spawned in SA, they appear as Varrios Los Aztecas. They can't be spawned in GTA III.
The gang doesn't appear in the game outside of using cheats or mods. As said, their appearance is identical to the Varrios Los Aztecas, with no changes in the look. They behaviour, however, is different - they won't attack Carl at first sight, even, they don't want to confront him.
Ways to spawn
The gang can be spawned in various ways, cheating being in all of them:
1.Put on the cheat BIFBUZZ (PC).Now all gangs should appear on the streets, underneath Aztecas driving Sentinels. These are members of the Unused Gang 9.
2.Various save game editors can be used to add a territory for the gangs, and have them in a district.
The gang can be customized, by changing they models to other ones and changing which cars they drive.A full tutorial about this can be seen here.