In the episode when Ash, Misty,  and Brock get sunk under water in a boat the man ash battled with a Raticate offered to trade Raticate for Butterfree. Ash went with the offer but kept it for like two minutes. So Ash didn't exactly own it.

In the episode when Ash saves a man from Zubat While they were in Mt. Moon battling Team rocket Ash finds out Brock caught a Zubat and Ash say "Aww I should of caught one." If he would of thought of that sooner he would have a Zubat.

 Again while they were in my. Moon they encounter a Clefairy which if that Study guy Professer what ever wasn't there to tell him not to catch Clefairy He would have a Clefairy (Maybe Not because It's so freaking rare and strong)