The pilot reported that, during his normal preflight procedures, he latched, but did not lock, the baggage door, and there were no problems with the latch. During the takeoff roll, at rotation speed, his wife, who was in the right cockpit seat, noticed that the baggage door had opened. Fearing that his dogs in the rear seat would exit the airplane, he decided to abort the takeoff. The airplane departed the end of the runway, through the overrun, and down an embankment before colliding with a perimeter fence, resulting in substantial damage to the right wing. A post-accident examination of the baggage door and latching hardware revealed no malfunctions or anomalies that would have precluded normal operation. There was a placard inside the cabin that required locking the baggage door as the last step to securing it.