When the lords of Sinnoh marched south to Kanto in an ancient attempt at building empire, they brought with them numerous Rhyhorn in their army. Until the age of concrete and steel, there would be built no walls or forts which could withstand a charging Rhyhorn, and few pokemon enjoyed getting in their way either; they were feared as a cavalry deadlier than Rapidash. (Rhydon, their evolved form, are bipeds who can not accomplish a charge.)
For all the fear they caused, Rhyhorn were not without their weaknesses, and a series of tactics employed by a general of that time exploited these flaws and saved Kanto from conquest and ruin. They are poor at turning, worse at stopping, and nothing exceptional in terms of speed, so there was ample time to calculate the trajectory at which one would attack: water guns properly employed often knocked them out before they ever reached their target. Dugtrio were used as well, with pitfall traps engineered to sink under a Rhyhorn's heavy weight; far too often they merely charged out, but it slowed them up just enough. And finally, two grass pokemon would stand on either side of the charging horde and trip them with a Vine Whip, although their short legs meant that the maneuver required considerable skill to execute.
The army of Sinnoh was routed and retreated home in ignominious defeat, demanding only that Kanto not spread their anti-Rhyhorn tactics to neighboring lands. The general kept his word and was given a massive payment in gratitude, which he used to bribe the legislature of the time into appointing him as dictator.
Thus did the Republic of Kanto fall.