Here's why you won't find Riolu in the wild:
* It evolves from the egg: Riolu is a special Pokémon that evolves from a specific egg, not through a level-up or trade evolution. This egg was only obtainable through a special event.
How to get Riolu in those games:
1. Find the Manaphy Egg: You had to attend a specific event where you could receive the Manaphy Egg, usually at a gaming convention or a special Nintendo event.
2. Hatch the egg: The Manaphy egg would hatch into a Manaphy.
3. Trade the Manaphy: Trading Manaphy to a friend's game would allow them to access the "Riolu Egg" in their game. This egg would hatch into a Riolu.
Getting Riolu in Later DS Games:
* Pokémon HeartGold and SoulSilver: While Riolu wasn't available in the main game, it could be obtained through events, similar to the Diamond and Pearl scenario.
* Pokémon Black and White: Riolu was available as a regular Pokémon in the wild in these games, making it easier to find.
If you're looking for Riolu in a DS game, you'll need to find a version that has it available as a regular Pokémon in the wild or through an event. You may also be able to trade with someone who has a Riolu in a later generation game.