Ash has in fact caught every starter Pokémon in Kanto, Johto, and Unova, as well as catching every regional bird except for Toucannon in Alola. But there have been several of Ash's team members that no one expected he would ever catch, even in the episode in which he catches that Pokémon.
Updated on September 14, 2022 by Daniel Kurland: The enthusiasm that surrounds Pokémon as a whole has only grown more intense over time as the franchise invades and dominates new media. The Pokémon anime alone has produced more than 1000 episodes and two-dozen feature films over the span of 25 years. Ash’s position as Pokémon’s hero remains the constant through the franchise, which has helped him capture some impressive Pokémon along the way. Ash is quite selective when it comes to the Pokémon that he captures and he hasn’t even acquired 100 unique creatures. However, some of the Pokémon in Ash’s party are truly surprising selections.
Muk is a classic Pokémon that harkens all the way back to the Kanto region's original 151 Pokémon. Muk isn't a Legendary or Mythical creature, and it's not a Pokémon that causes much of a stir in the video games. However, Ash's Muk is a more unique situation and the grimy Pokémon forms a codependent attachment with its trainer.
Ash has no interest in catching a Muk and this act happens purely by chance. His priority at the moment is to clear out an unruly pack of Grimer. But his efforts to ward off their leader, Muk, confine the creature to Ash's Pokéball. Ash first shows signs of embarrassment over his Muk, especially since it loves to glom onto him and occasionally swallow him whole. However, it has also turned into a valuable battle asset in Kanto and Johto.