Except most dont in the series. Too be fair, most of the time the pokemon we see ash catch are plot devices like the head of the squirtle squad, or the abused charmander he saves, and they choose to come with him willingly. But honestly, the only one I can think of offhand that wasnt taken willingly, was probably caterpie, and after evolving fully, ASH LET HIM GO. His butterfree found a mate, and instead of saying, "Sucks to be you, lets go find us a pokemon to beat up" Ash, sobbing his eyes out, let him go so he could be happy and have a family or whatever.
Basically, the entire point of ash in the series to show us that friendship is magic. He makes friends with all his pokemon, helps them achieve THEIR goals, like primape wanting to be a butt whuppingly strong fighter, or charizard and his trip to the reserve, pikachu wanting to kickass forever without evolving for whatever reason he had, things like that. Yes he wants to be the greatest pokemon trainer in the world, but he also cares about what his pokemon want to do and helps them do it. In return, they get crushingly powerful, able to ignore type disadvanatges at will, and help ash become a champ.