before mount Everest was discovered, what is the tallest mountain?

Mount Everest isn't the tallest mountain on Earth. It's the highest. The tallest was and is Mauna Kea in Hawaii at over ten thousand metres.

Mount Everest was still the highest mountain. Just that nobody knew about it!

If you mean which is the second highest mountain the answer is K2 - another mountain in the same ranges.

edit
in response and clarification to KTDykes
What is the world's tallest mountain from base to peak?
Mauna Kea in Hawaii is the world's tallest mountain from base to peak! Its base is on the sea floor and it rises 33,480 feet (10,314 meters) in total, reaching 13,796 feet (4205 m) above sea level.

While Mt. Everest is the world's tallest mountain above sea level, Mauna Kea is still quite spectacular

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Tian Shan in Xinjiang in northwestern China, Kazakhstan, Kyrgyzstan, Tajikistan, and Uzbekistan.