From New Bark to Olivine, Raikou races across Johto, like its siblings running day and night, awake and asleep, across land and water. Unlike the fiery Entei and naturally aquatic Suicune, however a Raikou's journeys across water are a source for immense terror; an old water pokemon in Johto is often defined as one who has learned to avoid its many routes. Water, after all, conducts electricity, and Raikou offer plenty of volts to conduct.
It is generally held that Raikou run because they can not stop, for much as Kanto's weather is dependent on the legendary birds only rarely venturing from their lairs, so is Johto's on the legendary beasts racing constantly across the land; the storms they summon keep worse chaos at bay. But many, citing the constant storms which batter Cianwood and the Whirl Islands, are unconvinced that Raikou need venture out so far at all. Instead, they claim that Raikou, like Meowth and other feline pokemon, are said to love the taste of fish, and a legendary pokemon has a legendary appetite. (Needless to say, this theory is rejected by those who favor a canine explanation for Raikou's origins.)
Raikou, however, has not existed forever, but was born when Ecruteak's tower burned. Although most claim Ho-oh regulated the weather before the tower burned, some have used this fact to deny that Raikou have anything to do with the weather at all. They say that Raikou neither accidentally electrocutes pokemon, nor fishes with lightning, but targets the sea because, in another life, it saw a sailor on leave burn down the Brass Tower and swore revenge against everything he loved.
Whatever reason they run, nowhere along Johto's coast do people forget to leave offerings and pray for Raikou and its thunder to stay far away from them.