Sand W Sand Worm
Sand Worms are found alone or in pairs and only in the later parts of the game (usually in deserts).
They use the powerful CRACK spell, (basically the same as QAKE), which can kill your whole party at once. Run the heck away, especially if you don't have some Ribbons. A Ribbon is the only thing to defend against Crack so you're likely to end up with one dead party member if you do fight.
Sand Worm is the same in every version. I don't know of any origin. They reappear in FF2, FF3, FF4, FF5, FF10, and Mystic Quest. For more worm-related info head to the Worm page.