Have your vet look at it. He'll be able to tell wether it is a cyst or a tumor. Most of the time it's free just to get it looked at. (though some vets charge $40 for walking in the door...) If it's just a cyst, you have the option of leaving it or removing it.. If it's a tumor, he'll likely want to do a biopsy. (just removes a small tid bit to see what it is) If it's cancerous, you'll deffinately want it removed and then some. If it's just a benign fatty tumor, they'll give you the option of removal or leaving it be. my vet says removing fatty tumors only "cheese (he used a different word starting with P) it off" And that they usually just come back..bigger than the first time so it's better to just leave it alone. Regaurdless of what it is, you should deffinately see a vet.
Vet tech student. Expericence w/ my dogs and vet