If the domestication of Oddish and Miltank created civilization, the domestication of Steelix created empires. Wars in the Bronze Age, before pokeballs and most domestic pokemon, were fought with weapons: spears, shields, javelins, and chariots pulled by Ponyta. For a time crawling behind trained Onix was a common tactic to avoid ranged attacks, similar to more recent modes of trench warfare. Onix, however, could not move easily in the grasslands and flood plains where many wars were fought, and their thin joints rendered them vulnerable targets, so their effectiveness was limited; they could not protect many men, and the valuable Onix were themselves often slain.
And then came the Iron Age, which is often dated not from the use of iron tools, but from the application of metal coats to Onix, creating the first Steelix. Larger, stronger, and most importantly, sturdier than their predecessors, not only could a hundred men advance behind them (if the old historians are to be believed), but they could break a chariot with a single thrash of their tail, and spears bounced off them like thrown rocks. Armies declined and became abandoned outright for a time, for the only thing which could beat a Steelix was another Steelix, and a community's military power became based on access to Onix caves and iron.
Thus human battles were replaced by pokemon battles. Centuries later, great upheavals occurred as the first grass and water pokemon were domesticated and a tactical revolution swept the world. Steelix, despite their strong physical defenses, were slain in war and abandoned in peace. The modern population of Steelix descends not from the Steelix champions of the Iron Age, but from wild Onix fashioning coats of their own from scrap metal to protect themselves from predators; ancient Steelix are known only from ruins.