Mossy Cave

Located in Johto. You need to have completed the Collector lll Achievement (press the gold/white ribbon button on the bottom left to see achievements) to receive the Mossy Key. The actual Mossy Cave is found in The Dragons Den. Mossy Cave is in Johto. You surf upwards from the gym in Blackthorn city. It 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 within the MC. Entei is in the Fire Cavern(left), Suicune in the Water Cavern(middle) and Raikou in the Thunder Cavern on the right.

(As of Apr 7 2018)

Additional information
The following fossil pokemon can be found in Mossy Cave (in all caverns):

Shard drop rate: 1/60