The pilot was departing a private airstrip toward the south, and had just obtained lift-off airspeed. The passenger and pilot noticed an unidentified object run across the runway under the main wheels. The pilot thought the object was a dog or child. The pilot abruptly pulled the control stick aft. At the same time, the passenger depressed the right rudder. Both actions were an attempt to miss hitting the object. The right wing struck the runway, and the airplane ground looped and nosed over.