Well it could be a few things, do you live in the city, urban areas, wetlands, woods, ect..
It could be a groundhog, rabbitts, skunk, oppsoum, ground squirrels, fox, badgers, vole, chipmunks. in florida, my friends live near wetland. Guess what they get?
Your dogs can go into a frenzy, and dig and dig, till they get it. or worse, if a badger get hurt. Call animal control and see if you can have them set a trap, if not buy one on ebay. We had to. If you get a skunk, oh boy. we ended up carefully wrapping a blanket around it. (the trap) duck taped it. Then took it to the woods. As careful as we tryed. Never cryed so much in my life, I'll never forget that day. or, the trip again 3 months later, because we forgot to bury that hole up. A new skunk neighbor moved in again.