Mighty mouse on roids! hahaha its great! I really like the model. It has a lot of character and the head/face is really well done, ears too. Nice sculpting work!
Looks like you used muscle anatomy reference which is good, however you forgot about skin and fat which is almost always stripped away in muscle ref. It looks like he has a negative body fat ratio, rats are lazy scavengers who carry quite a bit of fat for such a small animal. Also note, when body builders have this level of muscle detail (they really don't get this well defined, they would die first) they are actually at their weakest. If your rat is going to be a warrior he wouldn't starve himself for days and be at the point of almost clasping, all the time. Body builders do that for a week ramping up to a competition and it is very damaging, but looks impressive.
Muscle bulk and definition do not always = strength. I would suggest put some fat/skin back on him in some key areas.
Tails, WHY does everyone make tails jut out from the tail bone at a 90 degree angle? Its an extension of the spine/tail bone. Just because we can stand a character up right doesn't mean we get to break their tails. I don't buy the excuse its for rigging either, at a 90 degree angle it just makes it hard to weight correctly where it counts and can cause stretching when you relax the tail.
Check the skeleton of just about anything with a tail if you think I'm nuts.