It is unknown what their original homeworld was, assuming they had one, but given their size, they likely lived on a very large planet.
Only one of their kind was ever encountered by the Super Friends, it was the Moon creature that hatched from the center of Earth's moon. Presumably one of the parents of this creature had placed it in the core of the moon, possibly to incubate it. This would have had to have occurred sometime in the 1980s, because prior to that, it was learned that the creature was indeed not in the center of the moon. One other possibility is that the creature was not fully developed until the '80s, meaning it may have still been an embryo during the period in which Sinestro created a giant ray gun to cut the moon in two, and somehow managed to survive, by being inside of one of the halves of the moon.