Hunted nearly to extinction during 20th century whaling, the Antarctic blue whale, the world's largest animal, went from a population size of roughly 200,000 to little more than 300. The most recent estimate in 2004 put Antarctic blue whales at less than 1% of their pre-whaling levels.
TLDR:
If we somehow let this magnificent animal go extinct, it will be the ultimate symbol of our failure as species.