I've known 2 dogs that do this. 1 was a Flat Coat Retriever named Corner. He was GREAAT! So, much fun. The 2nd I just met. Her name is Otter. She is 1/3 Rottie, 1/3Chow, /1/3 Pit. She is the SWEETEST thing! It was VERY unnerving when she did this though because she "Looked" menacing.