This npc is part of the Northern Exposure and Frostbitten achievements. He drops Calcified Web Spaulders which is a level 74 bind on equip mail shoulder 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.
Terror Spinner is also a tamable hunter pet. Please note that because it is also a hunter pet that even though you may not have seen Terror Spinner before, it doesn't mean that it 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 this guy 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.
Terror Spinner only has a couple of abilities. They are Impairing poison, which slows casting by 50% and movement speed by 50% for 10 seconds, and Instant poison which deals nature damage. When tamed his special ability is a web which prevents targets from moving for 5 seconds.