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.
Old Crystalbark is pretty to easy to spot when he is up as he is much darker in color than the other tree giants around him. He also has huge pink crystal tusks.
He doesn't share a spawn timer with any other rares. I've only seen this npc up once, and I've quested with a lot of different toons in this area. I imagine that he usually doesn't stay up long when he spawns in as there are generally several people questing in the area he patrols through who will notice him.
Anyone questing that runs across him should have no issues killing him and will get a hefty chunk of experience for their efforts.