What Pokémon Did Ash Not Evolve?

Back in the initial season of the anime, Pikachu wasn't Ash's only Pokemon that didn't jump right into evolution. Ash eventually had one of each of the lines of Kanto starter Pokemon, Bulbasaur, Squirtle, and Charmander. Out of the three, both Squirtle and Bulbasaur have remained in their unevolved forms.

Why did Ash Pokémon never evolve?

Oshawott was with Ash for most of his journey in Unova, yet it just never evolved. It was a vital member of Ash's team during the Black and White seasons, though he admittedly acted like a coward sometimes when he thought he couldn't win a fight. It's possible it was his own cowardness that prevented him from evolving.

Why doesn't Ash's Bulbasaur and Squirtle evolve?

One Reddit user theorized that the reason Bulbasaur and Squirtle chose not to evolve is because of Charizard. As a Charmander, it was clear to see that they were friends -- the episode "Island of the Giant Pokémon" is given as an example of this friendship.