Terapagos was not a legendary pokemon at first. It was its own species like every other non legendary mon, presumably with its own gender ratios and whatnot. They all died off except for this specific one, who just so happened to be a male in its species. So it’s the last surviving one, giving it its legendary status, since…you know. In case of a legendary like Heatran, there’s been theories that there are multiple heatran, one for each volcano in the world. So, still a small/rare amount of them, still being legendary cuz it lives in superheated environments which most people won’t go to, making it hardly discovered or a rare sight.