It is located in the submarine south of where strength is required. To get back here, you must enter the eastern entrance to Madio Cave and down the first ladder. Go all the way to the other side and you will see a ladder, if you have Strength, you can pass. This will take you to a different part of Littleroot Town (this is the only way to access this bit.) Walk to the small pool of water and you will re-enter the submarine. After defeat or capture (preferably capture), your character will be teleported back to Oldale Town.
It knows Will-o-Wisp, Seismic Toss, Rest, and Sleep Talk at level 66, when it's captured.
Previously known moves: Self-Destruct, Ember, Fire Blast, Iron Defense, Role Play.