There are a few like Rotom, that can only be obtained (as in X/Y) only by certain events like finding it in the lost hotel's dustbins.
Other than these, most in-game legendaries are also specially obtained as interactive Pokemon, (non-legendary interactive Pokemon iclude the red Gyarados, shiny Haxorus and Snorlax).
Roaming legends also need special conditions fulfilled to be found in the wild.
Hope I helped!