At the back of Route 16, there is a Mysterious Entity (Ghostlike Trainer) that you can battle. Keep in mind that its Pokémon are very strong (With the average Pokémon level of 100), so you'll need to prepare hard before you challenge the entity. When defeated, you will be rewarded with $23,000 Pokédollars. There are many Mysterious Entities are in the game.
Rarities: Common, Uncommon, Rare, Very Rare and Legendary