Harvest whale
From Halopedia, the Halo wiki
The Harvest Whale was a marine animal native to the planet Harvest.
It is rather more eel-like in appearance than normal and common Earth whales, and is unlikely to be a direct relative of Earth's mammalian variants.
When Harvest was glassed, the human population suffered tremendous loss, but the natural environment fared far worse. The entire ecosystem was turned to a toxic environment, leading many native species to die off or be slaughtered by the open toxic air. The Harvest Whales suffered such a fate. The fate of the Harvest Whales and the animals on the planet harvest is unknown, though most likely dead due to the environment.