It doesn't need to have a specific nature (adamant would be nice but I can go through the trouble of breeding that myself later) - I have no specific requirements, I just have pearl and would really like to get a Larvitar into my game so that I can breed a good one myself.
I can trade any of the Sinnoh or Hoenn starters if you need one of them, and I have quite a few Eevees of various natures if you'd like me to check for one that's specific.