Dog gives the user an increased sense of hearing and smell.
For Ryo, his sense of smell was to the point where he could tell how many people there were inside a forest from outside. It also gives him the characteristics of a dog, including paws, claws, fur and a long, black nose. Nothing else is known about his Quirk, but it is likely that it might increase his reflexes and strength.
Gigantomachia's version also grants him uncanny senses of hearing and smell, being able to detect and track down his master from miles away.