From Halopedia, the Halo wiki
The Eel-Snake Creature is a reptilian creature native to Harvest, a setting in the game Halo Wars. Its function was to act as what Ensemble Studios refer to as ambient life, or wild animals put in the game to make it seem more realistic. It, along with other ambient life, was actually cut from the final game.
It has a sharp, possibly barbed, tail and scales on its lower head which form a hardened carapace around the rest of its body.
It looks similar to the Harvest whale despite the color, and may or may not be the same creature.