He is named after the Pack rat, small rodents who build large complex nests using various available materials, due to the large rat sitting atop his head.
Finding Pack Rat varies between people's games, he can be located in the town center's sewers behind the movie theater but sometimes won't appear for some people. If Pack Rat isn't in his sewers start a skirmish around the museum to try to bring him out, keep fighting enemies until the map spawns him in and if he doesn't spawn reload the map and try again.