I believe that there are many dogs. These dogs are arranged non-hierarchically into categories known as “breeds,” each with their own appearance and temperament (though by no means are all dogs within a breed identical, and many rumors concerning each breed are in fact false).
It’s no coincidence that so many of our linguistic expressions contain references to these dogs (such as “dog gone it”). Moreover, I think we have all experienced otherwise inexplicable—maybe even implausible—acts of dog, such as when a dog eats your homework. Nor is it any coincidence that people who want to better themselves are attempting to “run with the big dogs.”
But while language is one thing, one might wonder if there is any empirical evidence for the existence of a dog or dogs. I think there is. Obviously, the various photos one can find online can be explained away as digital manipulations, so I won’t even try to use them to convince you. If you are just willing to go out for a walk, however, I think you are likely to find that dogs are all around you (unless you live in a particularly dogless place).
I guess my real recommendation is to go find a shelter and ask them to tell you about their dogs. But in the end, it’s up to you to allow dogs into your heart and into your life.