While the bacteria content of every animal's mouth is unknown, there has been plenty of speculation about which animal has the cleanest mouth. Although it may seem easy to compare, according to an interview on Live Science, comparing different species' bacteria content is like comparing apples to oranges.
Due to the fact that all species have different breeds of bacteria that are able to survive in their saliva, it would be extremely hard to answer the question of which animal has the cleanest mouth. There's a myth that dogs have the cleanest mouth, perhaps even cleaner than humans. However, recent research has proven that to be untrue.