Acquire a certain amount of "pug points" to unlock new characters, which can be selected from "character select" in the main menu.

Unlockable	How to unlock
Alien Pug	5,000 Pug Points.
Angel Pug	10,000 Pug Points.
Baby Rex	400 Pug Points.
Demon Pug	10,000 Pug Points.
Gringo Pug	150 Pug Points.
Neo Hamster	1,200 Pug Points.
Neo Panda	700 Pug Points.
Nyan Pug	2,500 Pug Points.
Pterodactyl	2,000 Pug Points.
Super Christmas Pug	50 Pug Points.