Dog becomes extremely defensive of hole he dug, refuses to leave it

Rigby is a beagle chihuahua mix. When he is on a walk, he is all beagle! He is constantly hunting for rabbits and small animals. He must have heard something underground and starting digging. No one could get near him or his hole, if they did, he would whine and growl. It was all his!! He found it! He never did find what was in there. But it sure was funny. Crazy dog!