Spoilers below. If you're playing the game for the first time, please refrain from looking at these.
All legendaries and their location requirements!
Celebi: Celebi is a 0.3 exclusive Pokémon. In Mystery gift there's still GS Ball which is still obtainable. Afterwards Forest Ruins 2nd underground floor.
Regice: Seafoam Islands (requires HM Surf and Waterfall)
Regirock: Victory Road (right side path between non smashable rocks but before Indigo League and need to search for a switch)
Registeel: Moonside Lake (requiures HM Surf and Dive)
Regigigas: Mt. Moon 2nd entrance (all 3 Regis in party + HM Rock Smash, Cut, Surf, Waterfall, Dive and TM Flash)
Ho-Oh: South Amber Road
Lugia: The Dead End (I think...)
Articuno Roaming in Runko after defeating Elite 4
Zapdos See Articuno requirements
Moltres Same with this one too ;)
Latias: Trainer Tower Gauntlet Mode victory in under 950 seconds (walking not counting)
Latios: Trainer Tower "Hard"core Mode victory in under 250 seconds
Keldeo: Final cave (probably in 2nd upper floor)
Entei: Roaming in Runnku Islands after clearing Final cave
Raikou: Roaming in Runnku Islands (same as Entei)
Suicune: Roaming in Runnku Islands (you get the idea right?)
Deoxys: Get 200'000 TP (Trainer points) reward is your ticket to obtain Deoxys ;)
Mew: Requires Old Sea Map (probably requires finished main story but can't remember)
Arceus: Mallet Town :) (note: as of right now it's either a Pokedex Reward or something's behind the door in Your room. If it's neither then I've no freaking idea)
And about roaming pokemens... I know for a fact that atleast Zapdos as well is capturable in route 19 in Runko but have no idea about others since I'm in the middle of clearing final cave. If had more free time and patience could easily tweek each route till find one :D
Also thanks to Savordez about most pokemon location and roaming route requirements and also thanks for Pokemon UE Wikia (note: all obtainable Pokemon page)
Happy legen (wait for it.... ) dairy hunting! ;)