It may also be inspired by Ananta Shesha, the king of Nagas in Hindu mythology who is usually portrayed as a giant, coiled serpent with a thousand heads capable of spewing venom and fire...
Additionally, it leeches energy to take for itself, and while that's shaky logic, it sounds like something a Poison type would do. It came to earth from a meteorite, so it could have absorbed something from that meteorite, who knows. That's pretty weak, but so is most logic in relation to Game Freak.
Maybe Game Freak just wanted a Poison type legendary.
Hope I helped!