Throughout Ash Ketchum's extensive journey, numerous Pokémon have been part of his adventure. However, some names sadly didn't get as much screen time as they deserved.
1. Heracross played the role of a trump card

Despite being the first Pokémon Ash captured in Johto, Heracross didn't get much spotlight in Ash's team. Its innate strength was evident from the beginning, but it was quickly relegated to Professor Oak's lab, staying mostly in the experimental room.
Heracross only rejoined the team when Ash needed it for fiery battles, such as in the Johto tournaments, Battle Frontier, and Sinnoh tournaments. While it had its moments, many felt sorry for Heracross for not being in more battles.
2. Kingler, sadly, never left the laboratory

Kanto region left behind many fascinating Pokémon as Ash made room for 3 starter Pokémon and Pikachu. Krabby is one such case, sent to Professor Oak as the team was already full. After that, Krabby rarely made appearances and was seldom seen.
Despite its limited screen time, it showcased potential in a few instances. It played a crucial role in helping Ash win his first battle in the Kanto tournament. Clearly, Ash gave this powerhouse a bit of an underappreciated treatment.
3. Goodra carried the team briefly and then lost steam

Goodra belonged to the pseudo-legendary group, one of the strongest Pokémon Ash ever caught. From a timid and weak Goomy, Ash quickly turned it into a powerhouse with tremendous strength. Thanks to Goodra's efforts, Ash secured a victory against Clemont. However, it seems that was Goodra's parting gift for Ash.
Just after 15 episodes, Goodra started seeing less action. Even when it made a comeback in the Kalos tournament, Goodra couldn't secure a single win. Clearly, compared to other Pokémon in the Kalos region, Goodra fell short.
4. Naganadel evolved without needing Ash's involvement

In the Sun & Moon series, Poipole showed immense fondness for Pikachu and agreed to stay with Ash for a while before being released. This marked the only time Ash befriended an Ultra Beast, a special extraterrestrial Pokémon akin to legends.
Unfortunately, Ash only used Poipole once before letting it go. Later, it evolved into Naganadel and had a standout battle with Professor Kukui. This highlights how Ash squandered its power.
5. Primeape came and went in a flash

Primeape is one of Ash's quirkiest 'acquisitions.' It appeared only twice, once when Ash caught it and once when Ash released it. Despite that, Primeape helped Ash win a Fighting-type Pokémon tournament.
Ash's opponents were so impressed with Primeape that they invited Ash to continue training. Since then, Primeape never returned to Ash. Another significant waste of potential among Kanto Pokémon.
6. Butterfree had an anticlimactic farewell despite deserving more screen time

Butterfree left a mark with many beautiful moments alongside Ash: the first Pokémon caught, the first Pokémon evolved, the first Pokémon set free. The episode 'Bye Bye Butterfree' remains memorable for fans due to the emotions it evoked.
While this was meaningful for Ash, Butterfree deserved to be developed into a more nuanced character. Despite bringing many emotions to Ash, it didn't have any achievements to its name and deserved more recognition before departing in such a manner.