I can't find a monster with such a name in the online guides. Perhaps you mean "rock slug" - those are found in Relleka Slayer Cave, and need salt to finish them off.
Cave goblins are found in the underground part of Lumbridge. You have to follow the markings in the caverns to find the cave goblins.
From Runescape Wikia: "The Cavefish are a type of fish that can only be found in the Living Rock Caverns. The Living Rock Caverns can be found underneath the Dwarven Mine..."
Cave eels may be found in the Lumbridge Caves.Cave eels require level 38 fishing to obtain as well as a cooking level of 38 in order to cook them.
Sometimes snails and slugs can be in the park but it's not really rare to be found in parks. Snails and Slugs can be found mostly in gardens.
Under Lumbridge, Beneath the swamp. There is a passage through the basement in the kitchen, and a hole in lumbridge swamp.
It is through the cave entrance under the Karamja volcano.
Slugs can be found on all continents, even Antarctica.
In the Fist of Guthix cave, which is in level 8 wilderness.
The duration of They Found a Cave is 1.02 hours.
RuneScape's website and the website of the creators of RuneScape can be found in the related links section.
In the gamers grotto. (The cave east of the Falador load stone)
Level two of the Stronghold of Security in Barbarian Village.