This paper explores the impact of an augmented reality (AR) virtual dog, designed to mimic user behavior, on the exercise experience in both solo and group settings. Focusing on the virtual pet's role as a companion during physical activity, we conducted a human-subject experiment comparing three conditions: a mimicking virtual dog, a randomly behaving virtual dog, and no virtual dog. Participants exercised either solo or in groups, specifically in pairs, allowing for a detailed analysis of how the behavior and physical presence of the virtual dog influenced users' exercise experience and social connections. The findings demonstrate that the mimicking virtual dog significantly enhanced the exercise experience, especially in solo settings, by fostering a stronger sense of companionship. In group exercises, the virtual dog acted as a social facilitator, improving group cohesion and interaction. This research highlights the potential of behavior-mimicking virtual pets to enhance both individual and group exercise experiences and offers valuable insights for developing AR-based fitness applications.