A dull dog... It's not particularly offensive or friendly. Generally it's a bad term, especially in phrases like dirty dog or just he/shes a dog (ugly). Some groups (black Americans I'm thinking of) use it in a positive sense such as he's my dog, or what's up dog, in which case it seems to be more like friend, but if you're not in this group, then don't use it this way.

Neither friendly nor offensive, I guess. For sake of security, I do not think you compare a dog to a real human being even if some western people like to have a dog at home.

I agree with Alex. FYI - "top dog" is a positive one. It means the top boss/manager, or the leader of an informal group. "Underdog" = in a competition, the person who or the team which is considered to be weaker