The Rat Tail is in the land of summons. Which is a cave in the underground. You can get to it when your airship has the ablity to fly over lava. While your in the cave make sure you have float and cast it on every floor to avoid damage. The Rat Tail is in a chest in the village. Yiu can also battle aganist Ashura and levithen there. But to beat Ashura you need wall.