Six-Shooters & Wagons.
Yellow Bellied - When the posse is getting the upper hand on no-good varmints they might be able to cause their foes to turn tail and run. In groups of 5 or more, when half the number are killed, the rest must make a saving throw or flee in fear. This is done again when the group is reduced to 1/3 or its original number. You might also have a group do this if their leader is taken down in spectacular fashion (blown up, taken out with one bullet, etc).