The Gila monster is a venomous desert lizard . A heavy , slow-moving lizard , up to 60 c long , the Gila monster is the only venomous lizard native to the United States and one of only two known species of venomous lizards in North America , the other being its close relative , the Mexican beaded lizard ( H. horridum ) . Though the Gila monster is venomous , its sluggish nature means that it represents little threat to humans .