Latimeria chalumnae - Coelacanth
The coelacanth was assumed extinct until 1938, since fossils of it had been found long before, yet hadn’t ever been caught by anyone who recognized it (it was known as the “gombessa” by the Comoro Islands fishermen, and was considered a worthless fish to be disgarded, as it tasted awful). It’s thought to have evolved over 400 million years ago, and were originally assumed to have gone extinct in the Late Cretaceous period.
The Search Beneath the Sea: The Story of the Coelacanth. J.L.B. Smith, 1956.