Grocklar is a level 74 rare elite giant with approximately 41k health. He can be found patrolling in four different areas in western Grizzly Hills. (I have indicated his patrol areas in green on the map below.) He moves very quickly through his patrol area and is quite large. He is one of four rare spawns in this zone. The others are Arcturis (spirit bear), Seething Hate and Syreian the Bonecarver.
Grocklar is part of the Northern Exposure and Frostbitten achievements. He drops Rock-Giant's Pinky Cover which are level 72 bind on equip mail gloves with a random enchant. Like other Northrend rares, he also drops Abandoned Adventurer's Satchel, which generally contains about 20 gold, a full stack of frostweave cloth, and often times a few crystallized elements. He's also worth a hefty amount of experience for people leveling in the area.
People who come across him while leveling in the area should take some extra precautions when fighting this guy. He hits very hard and his patrol area also takes him through a lot of mobs. Be careful to pull only him otherwise an extra mob or two may make the fight go downhill quickly. I'd also like to note that if your playing Horde pulling him into the guards at Conquest Hold will not help you, they will not assist. He is also immune to fear.