A dried up crusty penis. It would look like a wrinkely pruney finger after you were in the bathtub too long.
*Grandma was baking cookies one day when grandpa walked out of the bathroom stark naked. She exclaimed "By Golly! He has a Twat Dog!"
**After being in the hot tub for quite some time, Billy stepped out and was pantsed by a friend exposing his Twat Dog to everyone at the party.