Old Crystalbark is a hostile level 71-72 rare tree giant with approximately 14 -14.5k health. He can be found in Coldarra wandering around the Nexus in the Borean Tundra and is one of three rares in the zone. The other two are Fumblub Gearwind and Icehorn.
I have indicated his patrol location in blue on the map below. This npc is part of the Northern Exposure and Frostbitten achievements. He drops Crystalbark's Toenail which are level 70 bind on equip plate 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.