What Breed Is My Dog?
Your dog looks like a black Lab with a Pit face. Lab/Pit is one of the most commonly seen mixes. Great Danes are somewhat rare and Greyhounds are very rare in the US, unless they are retired racing dogs, which would be spayed/neutered before being adopted out...
What Breed Is My Dog?
Looks like a shepherd/pit mix. You bought this dog from a breeder? Do you not trust the breeder? Think they are lying? I dont quite understand. Why would a breeder be selling a random mix anyway?
What Breed Is My Dog?
That is a purebred yellow lab. Not sure why that man told you such a thing.