"I was taken aback to find one of these creatures standing guard outside my own vault on my last visit."
-- letter from Mallory Twiddle (DP1)
Mallory Twiddle wrote a letter to the Daily Prophet, complaining that Gringotts was using Sphinxes to guard its high security vaults (DP1)
Twiddle relates in the letter that he encountered one of these guardians and had spent a frustrating hour trying to figure out its riddles. Eventually, the Sphinx chased him out of the bank (DP1)
While the date printed on DP1 is 31 July 1998, the timeframe for these events is 1992-1993.