Ash Ketchum, the protagonist of the Pokémon anime and manga series is a Pokémon trainer who aims to become a leading Pokémon master, as he travels far and wide with his friends. Although his first 'pocket monster' was a Pikachu that has continued to share a deep bond with him, Ash has captured some interesting Pokémon like Bulbasaur, Primeape, and Pidgetto. However, it's the electric Pokémon Pikachu that is mostly associated with Ash.
Even though he encounters and captures various species, he uses only a few. Here are some of his iconic catches that he should definitely use more.
Muk
Ash caught Muk in one of his earliest adventures in the Kanto region. A sticky creature made of purple sludge, Muk is definitely not one of Ash's weakest finds. A Dark/Poison-type Pokémon, Muk can still be a good-enough ally and was once used by Ash in the Indigo League to defeat a Bellsprout (as per The Electric Tale of Pikachu manga).
Mostly in the anime, Muk can be found in Professor Oak's laboratory where Ash has kept it. It is a pretty dangerous Pokemon as it is seen as a living biohazard, leaking toxins that can easily kill all plant life in its vicinity.
Primeape
Primeape is another Pokémon that isn't heavily featured in the anime, even if it is popular on the show's merchandise. A Fighting-type Pokémon, he evolved from Mankey and was yet again captured in Kanto. In its introductory episode, a Mankey stole Ash's cap evolving into a Primeape.
Ash eventually used his Charmander to defeat capture Primeape. His Primeape then undertakes training under Anthony, the owner of the Fighting Spirit Gym, a top-tier facility for Fighting-type Pokémon.
Donphan
Donphan is a Ground-type Pokémon that resembles an elephant and lives in rough, mountainous landscapes. Evolving from a Pokémon called Phanpy, it has been featured for limited scenes but has mostly proved to be valuable. It has bravely battled Ash's own Bulbasaur (although losing eventually) and has defeated Team Rocket.
In the episode Reversing the Charges, Donphan even masters the damaging Hyper Beam move. Barring this episode, Ash has hardly used the Pokémon.
Butterfree
In the Kanto region, Ash had also caught a Caterpie that eventually evolved into a Butterfree. Ash didn't really get a chance to use the purple butterfly as it desires to fly away with its peers in the wild.
As Ash lets go of Butterfree with several other Butterfrees, he freed it from his Pokeball in what is considered to be one of the most heartwarming moments of the series. Ash's Butterfree eventually mates with a pink Butterfree in the episode Bye Bye Butterfree.
Boldore
Boldore is a Rock-type Pokémon, one of Ash's strongest discoveries. It looks pretty majestic with a bluish shell and red crystals on its back and front. The crystals carry so much power that they are enough to power a hundred dump trucks. In fact, two of its orange crystals are hardened forms of extremely powerful and renewable energy.
Even though Boldore has hardly been used properly, it's a surprising find as Ash hardly captures Rock-type Pokémon in the first place.
Squirtle
One of the most popular Pokémon in existence, Ash's Squirtle was the leader of the Squirtle Squad. his group was a collective of rogue Squirtles that were abandoned by their Trainers.
Ash left the Water-type Pokémon back to the Squad only to be reunited once in Ash's last duel with Brandon, the chief of the Battle Pyramid. Apart from being Ash's first Water Pokémon, Squirtle happens to be one of his few team members that have not evolved, considering Ash's limited usage of Squirtle. Otherwise, it usually evolves into Wartortle.
Lapras
Lapras is a dual-type Pokémon, serving as both a Water and Ice-type. It resembles primitive aquatic dinosaurs and boasts of water-absorbing abilities and tough armor.
Ash rescued a baby Lapras for a group of thugs and Team Rocket, eventually using it as a mode of transportation on the islands of the Orange archipelago. Much like Butterfree's case, Ash releases the Lapras back into its family. Eventually, Lapras ends up leading a herd off the coast of Tarroco Island. Considering Lapras's ability to adjust in aquatic environments and its loyalty, Ash could have made good use of Lapras, though letting it go was the right choice.
Krabby
As its name suggests, this Water-type Pokémon looks like a common hard-shell crab that lives mostly on beaches. Even though Ash caught the crustacean once, he placed it under Professor Oak's care just like in the case of Muk. Krabby's armor and its claws (known as Hyper Cutters) make it of good use in battles.
In Round One Begin, Ash finally uses Krabby to defeat an Exeggutor (a Grass/Psychotic-type Pokémon). Krabby ended up evolving into Kingler in this battle.
Buizel
Buizel resembles a weasel or sea otter with brownish-orange fur. The Water-type Pokémon was initially owned by the ten-year-old trainer Dawn. Dawn exchanged her Buizel for Ash's Aipom. Rather than Pokémon contests, Buizel went on to be more interested in Gym Battles.
Buizel is an expert when it comes to swimming, possessing an ability called Swift Swim. It can be used for such purposes while fighting with other Water Pokémons. However, its lack of interest in the contests can prove to be a challenge.
Tauros
Tauros is one of the strongest Pokémons in existence, possessing brute force and intimidation. Apart from its strengths like Body Slam and Hyper Beam, Tauros's weaknesses are never pointed out in the series. It usually engages in a frenzy by whipping its adversaries with its three tails.
The quadruped bull first appeared in a highly controversial episode that was banned in most of the regions (for its heavy usage of guns). In this episode EP035 (also known as Legend of Miniryu), a horde of thirty Tauros is caught by Ash.