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Hello. Thanks for the question.
It sounds like you may be seeing a histiocytoma. This is a common mass that we see in young dogs < 3 years of age. They can pop up anywhere, but they tend to be more common on the head/ neck area. They will usually go away on their own over 1-2 months without any treatment.
To know for sure, you'd need to do a needle aspirate or remove/ biopsy the lump. Most of the time in young dogs we just ignore it unless it's getting larger or causing issues.
I hope this helps, let me know if you have any other questions.