Small land-air robots with rapid response and field operation capabilities hold significant potential for emergency rescue and field exploration applications. However, achieving stable and efficient mode transitions, along with effective terrain adaptability, remains a challenge. This letter presents a novel land-air robot, Flybot, which combines a bicopter UAV with a dual active-wheel, self-balancing wheel-legged base. Unlike traditional air-ground robots, this paper designs a single-drive, five-link leg structure that enhances lateral stability and terrain adaptability. By optimizing the five-link joint, we minimize energy consumption in ground mode, resulting in improved efficiency. Additionally, this paper establishes a hybrid dynamics model to describe Flybot's mode transitions and introduces a novel hybrid transition controller that leverages acceleration and throttle data to determine transition states. Finally, the adaptability and reliability of the robot are verified by the ground obstacle crossing experiment and the air-ground continuous motion experiment in the field, and the effectiveness of the hybrid transition controller of the robot is demonstrated. The weight-to-power ratio of the Flybot in air mode is 5.18 g/W, and the weight-to-power ratio in ground mode is 309 g/W, indicating the high efficiency of the Flybot.