Pentheus. It was also the name of one of the Argonauts, a son of Hermes and Antianira. As with Echidna, the basic meaning is 'viper'.
The direct link with the Sown Man is not clear here, but for the Argonaut the name may refer to the heraldic white ribbons on Hermes' staff which were later mistaken for snakes as he was the herald to Hades. However, the first Echion mentioned here was, like all the Sown Men, planted in the ground (as a dragon's tooth) by Cadmus, and Cadmus was eventually turned into a snake by Ares, so perhaps there is after all a sinuous