The vet says she’s not too worried about it, she measured it, and we’ll keep an eye on it. From the shape, and size and location it’s probably a fatty tumor, which is a type of benign growth.
Fatty tumors (lipomas) are very common in golden retrievers. The vet can probably just aspirate it, but if it keeps coming back it can be removed very easily. My 11 year old golden retriever, Snoopy, had 18 (!) of them removed last summer. She looked like a patchwork quilt for a while, but bounced back from the surgery very quickly.