The rules are right there on page 175, but they don't actually come out and say it.
They're under the heading "cosmetic changes."
Look for another level 1 minion that's close to what you want, and simply reflavor it. I'd use the Goblin Cutter, changing the size to medium. Alternatively, I'd just use Human Rabble, maybe changing Mob Rule to a bonus point or two of damage when it has combat advantage.