How can I get my dog back from my ex-boyfriend/roommate?

Is the dog your property (i.e. did you buy him by yourself, or own him before moving in with your ex)? If so, the man has no right to keep the dog and you can, if necessary, sue to get the dog back, including by going to small claims. The real issue will just be proof--can you show it's your dog.

If the dog was a joint dog--you bought him together--then the situation is much more complex and you may not be able to get possession of the dog; you could still sue, on the grounds that you are part owner, but it's not a given that you will be able prevail and get the dog. (For example, it's possible you would win only your share of the dog's economic value.)

If the dog was bought by your ex, then even if you considered it your dog, legally it's his and he can keep it.