Located in Johto. You need to have completed the Collector lll Achievemen(catch 350 Pokemon)(press the gold/white ribbon button on the bottom left to see adchievements) to recieve the Mossy Key. The actual Mossy Cave is found in The Dragons Den. Mossy Cave is in Johto and is home to the 3 Legendary Dogs(2 cats and a dog, lol) Entei, Raikou, and Suicune. Also Yanma, Exeggcute, Ferroseed, Scyther, Pinser, Heracross, Skorupi, Klefei, Sandile, Chespin, Milotic, and a few more common/uncommons. The Legendaries are catchable in certain rooms w/in MC. Entei is in the Fire Cavern(left), Suicune in the Water Cavern(middle) and Raikou in the Thunder Cavern on the right.