His name is Arcturius. He is, indeed, a Spirit Beast — complete with all the Spirit Beast skill, including Prowl. Like other Spirit Beasts, he is a non-aggressive rare elite. Unlike them, he’s only level 74.
Hunter safrienaer tamed him in Grizzly Hills just north of Amberpine Lodge — you can find a map and more pictures on the forum post. safrienaer also notes that:
As of now he doesn’t have any special sound effects – he sounds just like any other bear.
But we probably have many builds left before Patch 3.3 is ready for prime time, so I wouldn’t be surprised if he ends up with unique sounds and possibly some more glowy effects as well.
We’ll have to keep a careful eye on this bear!
 And because I think it’s very cool, I’m reposting info that Jangalian dug up:
Side note – thought his name looked hella familiar, so I put it into Wiki. Here’s the first answer it gave me…
Arcturus (α Boo / α Boötis / Alpha Boötis) is the brightest star in the constellation Boötes. With a visual magnitude of −0.05, it is also the third brightest star in the night sky, after Sirius and Canopus. It is, however, fainter than the combined light of the two main components of Alpha Centauri, which are too close together for the eye to resolve as separate sources of light, making Arcturus appear to be the fourth brightest. It is the second brightest star visible from northern latitudes and the brightest star in the northern celestial hemisphere. The star is in the Local Interstellar Cloud.
An easy way to find Arcturus is to follow the arc of the handle of the Plough. By continuing in this path, one can find Spica (α Virginis) as well—hence the maxim, “Arc to Arcturus, then speed on to Spica.”
Other uses include the following –
- Arcturus, a star known for its brightness.
- Arcturus in works of fiction, for fictional references to the star
- Arcturus (planet)
- Arcturus (band), an avant-garde Norwegian metal band.
- Arcturus (steamship), a passenger ship of the Finland Steamship Company.
- Arcturus, Zimbabwe, a village in the province of Mashonaland East, Zimbabwe.
- Arcturus Mengsk, the leader of the Sons of Korhal and later Emperor of the Terran Dominion in the StarCraft computer game series.
- Project Arcturus, a failed Ancient power-source project in the Stargate Atlantis television series.
- Arcturus Group, a consultancy based in Brussels
=) Just FYI. And yeah, I know wiki changed the name, but there were no results for the original spelling. That’s the closest I could get.