Why Hercules? Because Hercules was known for his feats of strength, and the Ninth Planet may "hurl" the occasional comet at the inner solar system.
Keeping with the Roman names (Mars, Mercury, Jupiter, Venus, Vesta, Ceres, etc) would also fit well, with Hercules being the Roman form of Heracles.
I think one current favourite was Persephone, since she was 'kidnapped by Pluto', and brought to the Outer Solar System, or Terminus, god of boundaries.