The Spirit Beast is an exotic Ferocity pet. Only those hunters with the Beast Mastery talent can tame him. His special attack is Spirit Strike. It does immediate damage and then a delayed explosion of damage.There was the day we beat Lady Vashj for the first time, pre-nerf. In our jubilation, we totally high-fived a lamp to its destruction, and thus Mrs. BRK finally got rid of, "that hideous piece of junk".
Where does the Spirit Beast spawn? There are many "rumors" of known locations, but the places we can confirm he spawns -screenshots FTW -are shown on the map below. In the movie we say "two", but of course someone emailed us a pic of a third location -the northern one -after we finished the movie. We found our Spirit Beast at 21.7, 70.1, which is the most western spawn point. 
Mania, the goddess of Petopia, has a link to a map of spawn locations, but we haven't seen screenshots of the other two points on their map. Those may be good points, we just can't /BRK-Stamp them.
There are other rare pets in the game, but we are not aware of any that have such a long spawn timer, can spawn in multiple locations, and can only be used by those hunters choosing to go at least fifty-one points deep into the Beast Mastery tree. Taming a Spirit Beast is a tremendous challenge, one we're thrilled to have been lucky enough to have completed.
You are invited to download the YouTube version (45MB) of this movie here, and the full-sized version (70MB) here. Crank your speakers; these downloads are in stereo for the best sound reproduction.
The proper name of our Spirit Beast is Loque'nahak. He is the mate of the chained god Har'koa in Zul'Drak. (In the movie we call Spirit Beast a "her", as we assumed Har'koa being a "god" was a male. /fail.) He is a rare elite, and unfortunately any class who kills her gets the Northern Exposure achievement.
Dear lesser classes, please don't kill him, we hunters are begging you. Killing Loq makes us cry.