Ash has a habit of encountering the most popular of Pokemon, going on adventures with them, and then disappointing everyone watching by never following through with catching them -- the most basic element of the Pokemon franchise.
However, Season 23 of the Pokemon anime has been doing wonders with Ash as a trainer by actually allowing him to catch popular Pokemon from all sorts of generations, which is precisely why the Pokemon Riolu is trending.
Ash catches the little guy in the latest episode of the anime -- the 21st episode, to be exact -- and fans are losing it. Ash has caught some prime Pokemon this season, including a Dragonite, but if there is one Pokemon that Ash should have already had out of the "Dream Team" catches he's made so far, it would be Riolu.
There are several reasons that fans praise this episode, but the primary one has to be its pay-off factor. This one twenty-minute long episode pays off over a decade's worth of events that should have seen Ash catching a Riolu before. It's satisfying in so many ways, and just fun to watch overall.
For example, fans of the anime learned a long time ago that Ash was a rare human capable of controlling and interacting with aura, just like Riolu and Lucario can. However, the Pokemon anime never did anything significant with it until now. This episode sees Ash finally interacting with and playing around with his abilities, and its end promises to explore this concept more in the future.
This idea of pay-off is what made Ash catching a Gengar so exciting. It wasn't just the fact that Gengar is a cool Pokemon, but the fact that Ash had a history with both it and its pre-evolutions. He actually won his first Psychic-type Pokemon badge using a Haunter that wasn't even his.
Riolu follows this same concept of history, only to a greater extent. Ash has had way too many encounters with it and its evolution, Lucario, not to have one of his own. Some honorable mentions would include the two-part Diamond and Pearl special "Pokemon Ranger and the Kidnapped Riolu," and the movie Pokemon: Lucario and the Mystery of Mew.
With the way this season is shaping up, it'll be interesting to see what other "legacy" Pokemon Ash might catch. Some are hoping that he'll have a Legendary of his own before everything is said and done.