It's a CHA/level based skill, which after you hit GM, goes out the window. At GM you can tame anything tamable regardless of CHA. There are WAY more tamable pets in the game than people realize or admit because people give up trying to tame things after 10 minutes or more. The boulder bear, spirit stag and midnight panther are all tamable, but for a long time, people told me you could only tame the boulder bear, which was a lie. I'd point out some more obscure and less known pets, but I'm saving those for the next time I roll a scout.
I also think its based on natural CHA, but I don't really know and am just assuming. But it seemed to me that CHA gear never helped my tame success but natural CHA did. It may just be a perception thing though.