Hey CC - check out the areas you MTB to make sure they dont have any rules regarding dogs. I know some National and State Park areas do not allow dogs.
Also be aware Roos and Dogs dont mix real well so if you mountain bike where there are a lot of Roos you will need to be careful. Roos can do major damage to dogs (I know my sister-in-law is a vet) and it can be pretty nasty.

Good luck with the pooch hunt.