Twolegs are actually humans in real life. There are two synonoms for the word, Nofurs and Upwalkers. Twolegs are described as having pink, naked hands and bright colored pelts over their body, covering everything expect their face, feet and hands. They have a little fur on their heads and are described of being mad and crazy.
Two Twolegs, Bob and Joe, come to ThunderClan's camp and set it on fire. Bob chokes Flameshine, nearly murdering her. Firepelt attacks the young male, sending him into a flame. Firepelt is burned to death. The two Twolegs escape.