Appendix G: Legendary Pokemon & Your Dex - Pokemon Black
The biggest challenge to completing the Pokedex in every generation of the game has always come down to the Legendary Pokemon. In spite of the fact that dex entries for most of the Legendary Pokemon in the game are not required in order for the game itself to consider your dex complete, most Trainers view it as an issue and prefer to 'Catch 'em All' in order to consider a dex complete.
In the case of Pokemon Black and White the challenge is not with the Legendary Pokemon in the 5th generation, but rather it is mostly with the Legendary Pokemon from the previous generations, some of which will be very difficult to obtain since the events that are connected to them have come and gone long ago...
In general there are three ways for obtaining Legendary Pokemon, and they are (1) Attend the events that they are distributed at / properly obtain the Wondercard for those events, (2) Use a game editor or cheating device, and (3) Trade for them.
Obviously for most of the Pokemon option (1) is not an option at all... Those events have come and gone and will never return. A lot of players will choose option (2) not understanding that doing so can have nasty consequences.... For instance any edits to the game will alter its checksum number, which makes the cartridge ineligible for use in tourneys, events, contests, and for some Wondercard events. For most Trainers option (2) is no option at all!
In the end that leaves you with option (3) -- trade for them. That may sound like a long-shot approach but in reality it is a lot easier than you might be thinking. There are two avenues to seek out trades in -- person-to-person (meaning with someone you know in real life), and online trades via GTS and WiFi Linking. If you have a real life Trainer or Trainers to trade with than you are all set, if not you need to use GTS.
The basic requirements for obtaining Pokemon through GTS is that you must have the basic Pokedex Entry for the Pokemon you want to trade for. In other words you must have seen the Pokemon in the game at some point in order for the GTS system to allow you to seek trades for it. Since most of the previous gen Legendary Pokemon are not in the game, you will need to find other Trainers to do what are called Dex Trades for you.
A Dex Trade is exactly what it sounds like -- a trade in which you receive a Pokemon you do not have the dex entry for, and then send it right back to its owner. Most Trainers are willing to do this, in fact a lot of fan sites have chat boards specifically for this sort of thing.
Once you obtain the Dex Entry for the Legendary, you can then seek it in trades via GTS to obtain your own, and by doing it this way you will find that, while it takes some time and a lot of effort, completing your Dex is not the impossible challenge it often seems.
The following list is all of the Legendary Pokemon for each generation:
-- Generation 5: Black & White --
Those marked with a '*' are Legendary Pokemon you will have already caught in the game during your play through the story. Those marked with an '(e)' are event-only.
#494 Victini (e)
#647 Kerudio (e)
#648 Meloetta (e)
#649 Genosekuto (e)
-- Generation 4: Diamond & Pearl --
#491 Darkrai (e)
#490 Manaphy (e)
#492 Shaymin (e)
#493 Arceus (e)
-- Generation 3: Ruby & Sapphire --
#385 Jirachi (e)
#386 Deoxys (e)
-- Generation 2: HeartGold & SouldSilver --
#251 Celebi (e)
-- Generation 1: FireRed & LeafGreen --
#151 Mew (e)
The above list are the Pokemon you will want to arrange Dex Trades for, which means you will want to join a Pokemon Fan Site and begin establishing a good reputation there. Good luck!
Note: All of the above can be obtained via GTS trading once you have their dex entries except for #647 Kerudio, #648 Meloetta, and #649 Genosekuto, which have not been released yet. You will have to wait for their events.