Cobalion
>Cobalion is initially met on the way up through Route 6, having been made homeless from its previous home, the Mistralton Cave. Not long after this, it is simply found in the middle of Route 13, awaiting capture. If you defeat it and return after you have beaten the Elite Four, it is found here at a higher level.
Virizion
>Virizion, like Cobalion, is also found just standing around in the middle of a route having been made homeless. It will stand at the western end of Route 11, no matter how frozen the area gets. If you defeat it and return after you have beaten the Elite Four, it is found here at a higher level.
Terrakion
>Terrakion is hidden away on a little platform in Route 22. When you first approach it, Colress will appear and end up giving you his device for controlling Pokémon. If you defeat it and return after you have beaten the Elite Four, it is found here at a higher level.
Keldeo
Keldeo is an Event Pokémon and is not available through regular game play in Pokémon Black and White 2.
Note:
The sentence that is highlighted in bold at the end of the each paragraph under Cobalion, Virizion and Terrakion, which says "If you defeat it and return after you have beaten the Elite Four, it is found here at a higher level", this is also the case if you run away from it as well, so you just have to beat the Elite 4 for them to appear again.
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