*Ahem* Squee!!! XD;; That's so adorable. I love how they're so fluffy! I just wanna touch them. ;_;' That was the first thing I noticed and I had to get that out. The picture is very well done (as if we could expect any different from you ;p). I really like the way you did the lighting on the black Mareep's ears for some reason. I also really like the colouring job you did on their fur - it's really good. The only problems I see are in the normal Mareep's tail... it seems to be a little low and the fur is scarce in that area. The other thing I noticed are in the black Mareep's hind legs. The one we can see looks a little awkward and the other one seems to be missing. ^^; Other than that, the picture looks great.
Oooh... and I'm envious of your rock-drawing skills. >_>