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Pokémon that can only be obtained from an event (be it in-game or by a sponsored distribution of some sort) are listed below. You will find various types of data pertinent to these Pokémon including, but not limited to: how to obtain, level, moves known and stat ranges.

Roaming Pokémon can be encountered at any point in time in the wild after you have activated the event that permits them to roam. In HGSS you may even encounter these Pokémon while surfing.

Provided that Roamers have a tendency to want to escape from battles it is important to keep them right where you want them, in battle. Using Trapping moves is a good first step with Roamers, generally. Unfortunately for us Raikou and Entei know Roar and enjoy using it to make capturing them all that more difficult.

A friendly tip. To deal with the high volume of Roaming Pokémon in Platinum I bred an Absol and trained it specifically to capture those Pokémon. I bred a Jolly Absol with the learnset: Thunder Wave, Swords Dance, Mean Look and False Swipe. It can't actually KO anything but this is sort of the point. To get Mean Look you will need to breed Absol with an Umbreon that has learned it (this happens at level 57). Don't teach it Thunder Wave until it is at an acceptable level, so you can use that move slot to actually KO opponents and train it properly.

Johto's Beasts

You'll have only a small opportunity to observe Raikou, Entei and Suicune in Ecruteak's Brass Tower. Once you reach the bottom of the tower they will all flee. From this point on Raikou and Entei will be moving around Johto at level 40.

I'd suggest using the Master Ball on one of these two. Raikou has more PP, so it can theoretically endure more battles without being caught but it is also faster, which could make trapping it a bit more difficut.

Hoenn's Eon Pokémon

You will first need to obtain the Lost Item to activate the mini-event required to get the Eon Pokémon specific to your game to roam Johto. To do this you will first need to have the National Dex. Once you've arrived in Kanto via the S.S. Aqua, you have to head north to Cerulean City, and from there head east to Route 9. When you reach the end of the route, surf from the bend down to the Power Plant. Here, talk to some of the workers, then when you're leaving you'll be approached by one of them. They'll ask for your help in apprehending a criminal that has stolen a Machine Part. The thief is a leftover Team Rocket grunt you can find at Cerulean's Nugget Bridge. Defeat him, then follow his instructions to retrieve the part from Misty's gym. Next, head up to Saffron City and enter the pink house there, the home of the Copycat. On the second floor she will explain that she has lost her Clefairy Doll. Finally, head to the Fan Club in Vermillion City and you will be given the Lost Item to take back to the girl. Doing so will give you a Magnet Train Pass.

Having completed this, once you exit the Fan Club, Steven Stone of Hoenn will approach and explain that he is in Johto investigating why Lati@s has moved from Hoenn to Johto. He speculates that the reason is the presence of an elusive item known as the Soul Dew (for more details about this, click here). At this point you will be able capture the Lati@s that roams in your game. It will be much easier to capture than Raikou or Entei considering they do not know the move Roar.