Arcturis is a level 74 non-hostile rare spectral bear with approximately 15.4k hp. He can be found in Grizzly Hills in only one location which is just north of Amberpine Lodge (it is circled in blue on the map below.) He is also one of four rare spawns in this zone. The others are Grocklar, Seething Hate and Syreian the Bonecarver.
Arcturis is not part of any Northrend achievements. He drops Pristine Glowbear Pelt which is a level 72 bind on equip leather chest piece 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.
Arcturis is a tamable hunter pet. Please note that because he is a hunter pet that even though you may not have seen him before in Grizzly Hills 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!
Arcturis 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.
Arcturis doesn't really have many special abilities to note when fighting or taming him. He really is quite trivial, but I recommend leaving him alone so that hunters can tame him. When tamed his special ability is Roar of Courage, exotic ability is Spirit Mend and his bonus ability is Spirit Walk.
His spawn timer seems to be anywhere between 12 and 24 hours.