Grunter

Grunter is a level 57 aggressive rare yellow and black Felboar. He can be found just north of the Dark Portal in the Blasted lands. (I have indicated where he can be found in yellow on the map below.)  


He generally drops either an Imperfect Draenethyst Fragment or a Flawless Draenethyst Sphere, each of these items begins a quest. When turned in you will receive a "junk" bag for the imperfect one and a chest for the flawless. The junk bag usually contains a green item and the chest will have a blue. He will also usually drop the typical green boe and a decent amount of experience for people around his level.

He is also desirable as a hunter pet as his color-scheme isn't found very much throughout the world. The only other tamable boars in this color are the level 65-66 Domesticated Felboars in Shadowmoon Valley.

Grunter can be found with NPCScan. You will need to add it, his NPCScan ID is 8303.

You can also use this macro to help find and tame him:
/tar Grunter
/stopmacro [noexists]
/cast Tame Beast

I'm not sure on his exact spawn timer, but before Cataclysm it was fairy long, somewhere in the range of 32-48 hours. I believe is is still pretty long, but nowhere near where it was previously. I camped his spawn point on and off for several days before I finally found him. I think one of the main reasons that he may be a little harder to find is the location that he spawns in. It tends to be pretty highly traveled, even post-cataclysm.