Safari Zone themed parks have been a game staple since Red/Blue were released in 1996, in Japan. The fourth generation of games continues this tradition, however using a different moniker and changing the operation up.
In Sinnoh this park is referred to as the Great Marsh. What's different? This park, being a marsh, is very muddy. When traversing through the mud you may get stuck. To escape, you must rotate the directional keypad rapidly in whichever direction. You will eventually be freed but in doing this there is a good chance you will encounter a Pokémon.
The rapid change in direction may reflect a trick that was used by trainers in previous games relating to the step limitation. It was known that you didn't necessarily have to keep walking through the park to encounter a Pokémon. Instead you could conserve your steps to travel to all the areas by standing inplace and rotating about in the square you stood in. Its speculation that this is why this mechanic was officially adopted but logical nonetheless.
Another change is Daily Appearances, or rather a randomized rotation of Pokémon. By using the binoculars on the second floor of the Great Marsh building you will be able to see which Pokémon can be found in a certain area of the park. Sometimes you will be able to see a Pokémon that does not normally occur in park. We have a seperate list of those Pokémon provided here as well.
As well, Pokémon availability is limited before you get the National Dex. We have broken apart when Pokémon can be encountered and also indicated which versions of the games the Pokémon can be encountered in. Good luck!
Documented below are the Pokémon that can be found either in the grass or mud areas of the Great Marsh at any given time. There are no special circumstances related to encountering them, such as which PokéDex you have.
The following Pokémon can only be encountered while fishing. They are documented below.
(!) = Is only found in the water after obtaining the National Dex.
Sinnoh's Safari Zone, as it were, is different in operation from all the previous parks of similar operation. In Sinnoh there are Pokémon that will appear on random days. By using the binoculars that can be found in the second story of Great Marsh building you will have the opportunity to spot these Pokémon. The following list is comprised of the Pokémon that are in rotation before you get the National Dex. Note that some of the Pokémon listed above are also considered to be in rotation but it seems unnecessary to list them twice...
Above the Pokémon that appear in the Great Marsh on random days were listed if they appeared only before you get the National Dex. Realize that the listed Pokémon above are still in rotation here. After you get the National Dex, however, some new Pokémon are added to the randomized appearances list, and they are listed below.
(!) = The exclamation mark indicates a Pokémon that will only appear with the GameBoy Advance game FireRed inserted.