Hi,
It looks like a Histiocytoma. My dogs have had a couple of them. Histiocytoma's are a benign skin tumor. Histiocytoma's typically go away without any treatment, however it may take 2 or 3 months.
Both of my boxers have had these types of tumors, so don't be too worried about it, however...have the vet take a look at it, just to be sure, that is indeed what your dog has.
Best wishes...
Connie
I am not sure what that is but agree with Shannon that it should be checked.
Of course, if I ever question a suspicious lump, I am at the Vet's to get it checked.
That looks like something your vet should sample with a needle to make sure it is nothing serious; I hope not, but that should definitely be checked out!