The Thirsty Grunt is a Special Dialogue Grunt in the Halo: SPV3 level The Maw.
After the bridge where Foe Hammer is shot down, keep to the right side of the tunnel. You will eventually see a small passage in the side of the wall. Go through the passage and about halfway down, you will see another passage to your right. At the end of this passage will be the Thirsty Grunt. Upon approaching, he will say:
Good thing that Food Nipple's waiting for me at the starship 'cause man, whew, have I worked up a big, grunty thirst!
The description for the Grunt water bottle in the Bungie store mentions a "grunty thirst."
In Halo CE and CEA, the Thirsty Grunt carried a regular Plasma Pistol. In SPV3, he carries a Void's Tear, which is normally issued to Elite Zealots. How he obtained this is unknown.