There are currently 44 different Islands in The World of Portal Knights, each with their their various Creatures, Plants and Resources. There are at least 3 other Islands that are accessible only during special events.
Each island has its own save file in Portal Knights save file directory, and you can manually delete the file that matches the island to reset the island. The game will automatically recreate the deleted island once you travel there. Updated dungeon structures will be applied to the newly created island.