Skoll is a level 80 non-hostile rare mob with 18,900 health. He can be found in the Storm Peaks in several different locations which are indicated in blue below. This npc is not part of any achievements (most likely because he is a tamable hunter pet). He drops Skoll's Fang which is a level 78 bind on equip main hand dagger item 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.
Skoll is a tamable hunter pet. Please note that because he is a hunter pet that even though you may not have seen Skoll before in Storm Peaks it doesn't mean that he isn't already cached by NPCScan. If a hunter has this tamed pet out near you, your NPCScan will cache it. If it is cached in NPCScan and this rare is up your alert will not go off. Be sure to check and clear your cache often!
Also, please have some courtesy. If you come across this mob and a hunter is in the process of taming it - leave them be. Hunters, in order to lessen the chances of someone snagging Skoll while you are taming, hit him first so that you tag him. If a passer-by notices that the mob is grey'd out they are probably more likely to continue on their way.
Please note that because Skoll is a spirit beast, he can only be tamed by hunters that have the top-tier talent from the Beast Mastery tree Beast Mastery which allows the taming of exotic pets.
Skoll only has one special ability when fighting him, which is a pounce. He really is quite trivial, but I recommend leaving him alone so that hunters can tame him. (I did not kill him in the video below for that exact reason.) When tamed his special ability is Roar of Courage with the bonus ability Spirit Walk.