Here I will list off all the Pokémon Ash caught within the first 16 seasons of the anime, what season he caught them, what episode he caught them, where he caught them, and their original trainers (if they had a trainer before Ash). Here are the first 20.
Kanto (Original Series)
1. Pikachu (episode 1- Pokémon- I Choose You!)
Pikachu was Ash's starter Pokémon that he received from Professor Oak. He received Pikachu because he woke up late on the day he was supposed to start his Pokémon training. Pikachu didn't like Ash AT ALL when Ash first got him, and was very disobedient, refusing to help...