A box containing 100 gold. Which can also be exchanged with other players. Can be in all Lootboxes (Blue or Red) as a very rare item or as a random item in the 40€ LootBox Packet. Some bosses also have the Small Gold Box as a reward after defeating them.
You need the gold teeth to exchange them for HM06 Rock Smash. The NPC Warden has lost the gold teeth in the Safari Zone and can no longer speak a word, so you have to find the teeth and bring them back. You can find Warden in Fuchsia, the first house to the right of the PokéCenter. He will give you the quest "flealfglf mfmfl feeth".