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Gringott’s Dragon (more specifically part of the Ukrainian Ironbelly species) was used to guard several high-security vaults, including the Lestrange Vault, at Gringotts Wizarding Bank.
It was encountered by Harry Potter and his friends when they infiltrated the bank in 1998. (in order to obtain a horcrux) although it is likely the dragon had been guarding the vaults for a long time prior to this.