A Waddle to Remember Explorer Achievement takes place in Mount Maelstrom or Brisban Wildlands. Your aim is to find a chest. The chest have multiple possible spawn locations. When you find the chest, open it and Skritt Burglar will spawn. Follow him and try to kill him. The achievement is obtained upon being turned into a siamoth by Skritt Burglar. (Credit reddit community).
Wooded Kingdom Moons in Super Mario Odyssey