There are a variety of synonyms that can be used to describe a shy dog. Some possible options may include words like timid, introverted, bashful, withdrawn, reticent, or meek. These words all share a similar meaning in that they describe a dog that is hesitant or uncomfortable in social situations. Other synonyms that might be used to describe a shy dog could include words like apprehensive, cautious, reserved, or hesitant. Ultimately, the choice of synonym will depend on the context in which it is being used, as well as the specific personality and behavior of the dog in question.