Manaphy is an event Pokémon that is only obtainable in conjunction with the two Ranger games. As such, people who escape rope out of the ruins will not get to battle this Pokémon Although Defog is placed at the bottom of Solaceon Ruins, Unown does not appear to activate until you get to the bottom. No trainers have this alphabet Pokémon, and the one place that it inhabits is seldom visited. The first one that you likely will not see is Unown.
As such, here is a list of the more commonly missed Pokémon That said, there are several Pokémon that people will miss due to the fact they do not get used by trainers in the game or you do not automatically get their Pokédex entry when you progress through the story like Dialga & Palkia do. This is a very daunting task, but if you battle every trainer you see, you should be able to fill up your Pokédex as you travel through Sinnoh with very little difficulty. For a list of all Sinnoh Dex Pokémon, click here. However, Pokémon Platinum has a further 59 Pokémon in the Pokédex and you need to see all 210 of them as opposed to seeing the first 150 in Diamond & Pearl. This makes it easier than it was in the 3rd generation games. Like in Diamond & Pearl, this is done by merely seeing the Pokémon as opposed to capturing them. The method of getting the National Pokédex is to complete the Sinnoh Dex. The National Dex is a complete Pokédex capable of containing all the Pokémon from Bulbasaur to Arceus. Like the past games, you need to complete the Sinnoh Dex in order to get access to the National Dex. Like Pokémon Diamond & Pearl, the regional Pokédex and the areas around it are only the tip of the Sinnoh Iceberg.