#legendary classification

What makes a Pseudo-legendary different than a Legendary? For example: What makes a Dragonite different from a Lugia (aside from the size and typing, obviously), considering that they can be found in similar habitats (at least as Dratini and Dragonair) and the fact that both can, in their own way, manipulate weather (At least Dragonair can. Dragonite, I'm not sure)?

Is it legal to own a legendary that doesn't play a mandatory role in the world balance? Examples: Latios/Latias, Heatran, Shaymin, Zekrom/Reshiram... Possibly Shaymin is the easiest to take care of these...