Twin-Headed Tsuchinoko

Twin-Headed Tsuchinoko (双頭ツチノコ, Sōtō Tsuchinoko) is a Demon-level threat that resided under the Hero Association headquarters prison. He was killed by Saitama.

Appearance
Twin-Headed Tsuchinoko is a worm-like monster with two heads and very sharp teeth with no eyes.

Psychic Sisters Arc
During Tatsumaki's and Fubuki's battle, they damaged the area around them, causing Twin-Headed Tsuchinoko to escape prison. The monster sensed Saitama and tried to attack him, but Saitama simply grabbed the monster and smashed it into the ground, opening an underground prison of monsters.

Abilities and Powers
Not much is known about its powers, though given the fact it's a Demon-level threat, Twin-Headed Tsuchinoko is likely a powerful being.

Trivia

 * Tsuchinoko is a serpentine-cryptid or Yōkai with unique characteristics and is one of the most famous cryptids in Japan.