Rust makes the most sense. In the intro, you see ruins, but no sign of living civilization besides the little frog village. And in here, >>107672255
the big frog has scratched armor and a chipped sword. Most likely there is some sort of fallen frog civilization that will serve as the source of "lore" in this show. >>107673182
Plus, the box that transports them to Amphibia has three gemstones on it that correspond to the colors the girls wear.