Ash Ketchum has been the main protagonist of the Pokemon anime since it debuted so long ago. Throughout his journey, Ash has caught and traveled with a variety of Pokemon that he's encountered in various regions. However, many of them he had to leave behind in some capacity.
There are over 50 different Pokemon that Ash has travelled with in the Pokemon anime that are currently no longer with him. The majority of these Pokemon are currently staying with Professor Oak, while others are with other trusted friends of Ash or were formally released such as the heartbreaking moment he let loose Butterfree.
While there are no actual stats for Ash's Pokemon specifically, these Pokemon do have listed base stats from the mainline Pokemon games. These Pokemon also have documented win/loss ratios from Ash's many battles against other trainers and wild Pokemon.
Ash's Charizard
Other than Pikachu, Charizard is one of Ash's most well known Pokemon. Out of all of Ash's benched Pokemon, Charizard has stayed with Ash the longest with it also reappearing in many different seasons as a return or as a cameo.
Those who've watched the Pokemon anime from the beginning remember how Ash met Charizard back when Charizard was a Charmander. Back in the Kanto region, Charmander was abandoned by its original trainer named Damian. Still loyal to Damian, however, Charmander stayed where its trainer left it in hopes that he would come back for him. This almost cost Charmander its life when a rain storm came, threatening to extinguish its tail. Ash and friends, however, saved it and brought it back to a Pokemon Center. Damien eventually wanted Charmander back after seeing it help in the fight against Team Rocket, but Charmander decided to stay wish Ash instead and promptly used Flamethrower on its own trainer.
Charmander would eventually evolve into Charmeleon after helping stop a group of rampaging Exeggutor. While this did help the Pokemon grow stronger, this is also when Charmeleon started to no longer listen to Ash. This is to reflect the game mechanic of Pokemon of certain levels not obeying trainers that don't have enough badges. Still acting defiantly, though, Charmeleon did eventually also evolve into Ash's famous Charizard.
As Charizard, the Pokemon still would act disobedient and cause problems for Ash during his journey. Despite Charizard's behavior continuing, Ash took Charizard along with him to the Orange Islands, but only using it if necessary. However, one battle almost resulted in Charizard's tail getting doused as well as being encased in ice. Ash immediately did what he could to help warm Charizard up again and keep its tail lit, reminding Charizard of how the two first met. This helped solidify their bond and trust once more.
Charizard was eventually left with Liza in the Johto region for special training with it occasionally returning to Ash to help him once more. Charizard is currently with Professor Oak and many of Ash's other Pokemon.
Ash's Greninja
Ash's Greninja is famous for its power and it being a Pokemon to get an exclusive form from being with Ash known as Ash Greninja. Like with Charizard, Greninja had a stubborn personality except that it grew out of its defiance after evolving into Greninja. It also showed itself as being extremely protective of other Pokemon and people. As a Froakie, it was often returned to Professor Sycamore or abandoned due to its personality, but it grew close to Ash after he helped it with Froakie willingly being caught by the trainer.
Throughout the journey through the Kalos region, Froakie would eventually evolve into Frogadier and again into Greninja. Despite Greninja not being a Pokemon being able to normally experience Mega Evolution, the bond between Ash and Greninja allowed it to gain a new similar form being Ash Greninja.
This bond between Ash and Greninja has never been seen with any other Pokemon with not even Pikachu getting a new form from Ash's bond. Even the leader of Team Flare, Lysandre, tried to use this Bond Phenomena for his evil plans. By the time Ash's journey in Kalos had come to an end, Ash and Greninja went their separate ways with Greninja being released into the wild. This was done so that Greninja could help destroy what was left of the giant rock filled with Primal energy and help defend Kalos.
Ash's Krookodile
Out of all the Pokemon caught by Ash in the Unova region, Krookodile is definitely the strongest as well and the most stylish with it always wearing sunglasses. Ash found Krookodile as a Sandile out on Route 1 where he was causing trouble along with a group of other Pokemon. It was discovered tough that Sandile was trying to get the attention of the people on the route to a geyser soon erupting. Ash, however, did not catch Sandile with Sandile actually starting to stalk Ash due to a desire to battle Pikachu.
Sandile would continue to stalk Ash and friends in order to battle Pikachu on multiple occasions. It would even evolve into Krokorok while on this journey. After eventually getting that battle and losing, though, Krokorok became depressed. Ash then offered for the Pokemon to join his team, which the ground-type Pokemon happily accepted. Krokorok would eventually evolve into Krookodile.
In battle, Krookodile is a competitive beast and a powerhouse that has helped Ash win multiple Pokemon battles. It even was able to learn the move Aerial Ace in order to help take down a fighting-type Pokemon it was previously having a large disadvantage against.
After Ash's journey in the Unova region was over, Krookodile was left with Ash's other Pokemon at Professor Oak's lab. Ash then began his journey to the Kalos region.