For most of Japanese history, the region now known as Kanto was referred to as Azuma, after its population of Azumarill. This name came about because Azumarill bubbles were one of the region's most valuable exports, and also because with the rare Electabuzz their only natural predator in the area, their population exploded until they became as common as Tentacool and Tentacruel are today.
In the late Sengoku era, a lord who relied on misdirection through bubbles and powerful water attacks recruited whole battalions of Azumarill into his military to make use of his new tactic, depopulating the wilderness. He was victorious in battle after battle until becoming shogun in fact if not in name, and with his new conquest came endless castles to garrison. Azumarill were posted from castle to castle as garrisons, the bulk of his army coming with him when he moved to the capital in Ecruteak City. In time, as power corrupts, this lord became a tyrant who so feared rebellion that he forced all wild Azumarill into his army, believing that he could only be beaten by his own strategy. He especially feared rebellion from his own homeland, which was stripped bare of wild and trained Azumarill alike (except his own) and renamed to Kanto. And it was all for nothing, for when he died, no others could emulate his strategy, yet his successors maintained the policy out of terror for centuries.
The Azumarill garrisons were set free by lords who did not know how to use them; some populations took to the sea or died out, but the descendants of others survived to the present. Yet Azumarill, despite their ban, have not been forgotten in Kanto; they continue to appear in many hit television shows from the region such as Azumarill Daioh even today.