it is unknown
Once you move the rocks in the cavern you can find unknown in the Tanoby Ruins.
Piplup does not exist in Pokemon LeafGreen or FireRed.
No, you cannot find cyndaquil in LeafGreen.
you should go to gamestop
After entering the Hall of Fame at least once go to Pokemon Village from Snowbelle City. At the village look for some water to Surf on and use Surf. Go north and you will find a cave, this is Unknown Dungeon. Inside the cave you will find a lone Mewtwo waiting for you.
LeafGreen version does not have Growlithe.
You can't find Mewtwo in Pokémon Ruby. However, you can find it in another game and then trade. If you'd like to do this, you can find Mewtwo in the Cerulean Cave in the FireRed/LeafGreen game. In other generation games, Mewtwo can also be found in the Cerulean Cave in HeartGold/SoulSilver and in the Unknown Dungeon in X/Y.
In the grass...
You cannot catch houndour in Pokemon leafgreen either get one from Pokemon emerald or Pokemon colosseum.
there is no such Pokemon
That item doesn't exist in leafgreen.
you can't catch aipom in leafgreen but you can get one from Pokemon colosseum or Pokemon emerald.