Time for a serious response: I completely disagree with the people who have stated Mew-Two. Why you may ask? Because Mew-Two didn't ask to be created nor did it want to. Also because it is trying to figure out what its purpose is in life, he is trying to figure out his reason for existence. I highly doubt its purpose is to serve a dim witted 10 year old kid. Let's move on to Ho-oh, I believe Ash shouldn't capture it, I mean people assume Ash is the only one who has seen it but Morty saw it too right? I don't know, did he? and I'm sure other unknown people in the anime have seen it too. Ash seeing Ho-oh at the beginning of his journey was just a sign that Ash was going to accomplish great things in the near future. Next we have Latias, I feel like Latias could have been acceptable, but it goes against Ash and his beliefs to capture a legendary and the same applies for his corresponding lake guardian, Victini and Meloetta. In essence Ash will and should never capture a legendary because it goes against his own principles and beliefs and that's one of the things the anime inculcates to its target audience, that one should stand by his/her beliefs.