According to here, there is only one Legendary who is exclusively post-game.
Keldeo is obtained after you have rescued it from the Glacier Palace and finished the game
There is also Terrakion and Cobalion who ca nbe found at any time via random quests. The Forces of Nature are also obtainable randomly, so they can be found at any time. Finally, you can battle several "cover Legends" at random points via special quests, but they cannot be recruited. All of this information can be found in the above link.