And there ya go! I did it in Paint with nothing but a TouchPad, so please forgive the quality. Still, it details all known Mystery Dungeons in the world, and if (when ;) ) we decide to add more, I'll update it. For now, I'll now give a brief description of each dungeon, as well as a list of what Pokemon may be found there. It should also be noted that "Pokemon include" is a list of common and rare Pokemon, but you are free to encounter a Pokemon anywhere it would make logical sense. This is just a guideline.
Popla Forest is located just south of the town of Poplus, and is, in fact, the town's namesake. It's a very thin, woodsy area perfect for beginner explorers and low-level missions. The forest is often visited by townspeople to retrieve berries and apples, which grow in relative abundance throughout the forest.
Pokemon include: Budew, Bidoof, Ratatta, Sunkern, Wurmple, Pidgey, and Raticate (rare).
Difficulty: E-D
Crag Cave is rumored to have once been the stomping grounds of very powerful Pokemon, but if this is true, none of them still remain. The terrain is quite rocky, and the area is a favorite of Rock-type Pokemon, as well as a few who prefer dark places. Gravelrocks are easy to find here, but there is little else.
Pokemon include: Geodude, Tyrogue, Aron, Roggenrola, Woobat, Zubat, and Munna (rare).
Difficulty: E-D
Mount Mist is one of the smaller and more well-worn mountains. It has an easy path to the summit, as well as several side paths for the intrepid explorer. One can find a variety of hardy Pokemon while scaling this mountain, and it is rumored that strong Fighting-types occasionally use the summit to spar.
Pokemon include: Geodude, Pidove, Machop, Dwebble, Tyrunt (rare), and Axew (rare).
Difficulty: E-C
Breezy Meadow, as the name suggests, is a large, hilly area known for particularly pleasant weather. Grass-types often gather here to seek solace and relaxation. As such, many natural plants and their seeds can be found here, including apples and berries.
Pokemon include: Sunkern, Sunflora, Cherubi, Oddish, Hoppip, Sewaddle, Bulbasaur (rare)
Difficulty: D-C
The Smooth Plains are a flat, grassy area that stretch on for miles. A wide variety of Pokemon can be found wandering this area, but there is little in the way of food, so they rarely stay long.
Pokemon include: Skiploom, Bellsprout, Doduo, Durant, Tropius (rare), Salamence (rare)
Difficulty: D-C
Shimmer Beach, named for the way the sun shines on the water during the day, is a favorite vacation area. Pokemon from all across the known world come to relax and enjoy themselves here. While it is typically very small, it is still a known Mystery Dungeon.
Pokemon include: Seel, Poliwag, Luvdisc, Buizel, Corsola, Frillish, Mantine (rare), Water-type Starters (rare)
Difficulty: E-C (depending on what Pokemon happen to be there)
Named for the way the trees can block out the sun even in the daytime, Dusk Forest is home to many Grass and Dark-type Pokemon. Food here is scarce, due to the lack of sunlight, and even kind Pokemon can be highly protective of their territory.
Pokemon include: Poochyena, Mightyena, Purrloin, Liepard, Sableye, Absol (rare), Pawniard (rare)
Difficulty: D-B
Sapphire Lake, while not a Mystery Dungeon itself, can only be accessed by passing through Dusk Forest. It is known to be a common watering hole for many of the forest's Pokemon, and legend says that Suicune itself may occasionally appear here.
Mount Magmos is one of the primary landscapes that was rumored to have been affected by Giratina in ancient times. Once the largest mountain in the world, it was cleaved almost in half by a massive volcanic explosion, turning itself and the area around it into a charred and barren wasteland. It can only be accessed by crossing through the Ashlands.
Pokemon include: Houndour, Houndoom, Graveler, Magmar, Slugma, Camerupt (rare), Charizard (rare)
Difficulty: C-A
Rumored to have once been a beautiful mountain paradise, the Ashlands were completely destroyed by a disastrous eruption of Mount Magmos. Ashes rain down from the constantly-smoking volcano, and any plants that once grew here have long since been forgotten.
Pokemon include: Houndour, Litwick, Torkoal, Slugma, Numel, Charmeleon (rare), Pyroar (rare)
Difficulty: C-B
Legends say that the Mystic Sea is home to a mysterious Pokemon who sleeps beneath the waves, but no one has ever seen it. Still, it is easy to find powerful and impressive Pokemon here... if one can hold their breath.
Pokemon include: Tentacruel, Dratini, Wailmer, Gorebyss, Basculin, Phione (rare), Dragonair (rare)
Difficulty: C-A
The primary source of water that flows into the Mystic Sea, Rapid River boasts a fast-moving current and a lot of Water-type Pokemon. The lush trees around the river often drop their bounty into the waters.
Pokemon include: Magikarp, Poliwhirl, Alomomola, Lanturn, Pelliper, Lapras (rare), Marshtomp (rare)
Difficulty:D-A
A desert region in the southeast portion of the world, Dusty Dunes is home to many Ground, Rock, and Steel-type Pokemon. Travelers to this area should beware - sandstorms occur often and without warning, and there is little in the way of food or water.
Pokemon include: Hippopotus, Drilbur, Rhyhorn, Sandile, Carnivine, Krokorok, Krookodile (rare), Claydol (rare)
Difficulty: B-A
A long chasm that has been worn into the ground over hundreds of years. Researchers are conflicted about its origins, as it does not appear that such a crevice could form naturally. It is claimed a great monster sleeps in a hidden chamber at the chasm's base, but no one has ever found it.
Pokemon include: Marowak, Donphan, Hippowdon, Excadrill, Gliscore, Krookodile (rare), Golurk (rare)
Difficulty: B-S
A large mountain range at the northeast corner of the world, this area is home to many strong and dangerous Pokemon. Rockslides are not common, but also not unheard of, so travelers should be very careful. This mountain range is the only path to The Apex.
Pokemon include: Rhydon, Golem, Flygon, Donphan, Rampardos, Steelix, Tyranitar (rare), Aggron (rare), Rhyperior (rare)
Difficulty: A-S
An ancient and crumbling ruin that stands atop what legend says was an ancient battle ground against the Darkness. Ghost Pokemon are known to haunt this place in droves. To those brave enough to explore it, much history can be learned in this place.
Pokemon include: Banette, Dusclops, Yamask, Gengar, Spiritomb, Doublade, Aegislash (rare), Trevenant (rare), Shedinja (rare)
Difficulty: S-SS
The Apex is a mystical place that only the worthy are allowed to enter. This is the place that a Pokemon may learn the secrets of Mega Evolution. The Apex is able to alter itself specifically to face the Pokemon that seeks to be tested; it is very unlikely that any two Pokemon will face the same trials. It is unwise to come here alone.
Pokemon include: ???
Difficulty: SS
This mysterious entity appears in the desert at night, then disappears in an instant. Because of this, no one has ever gotten close enough to uncover what it is. Those that have gotten closest have reported hearing terrible noises and feeling very cold and sickly. Do not approach.
Pokemon include: ???
Difficulty: ???