Hi, I am Dr. Peter and would like to help. In response to your question, the best option is to have this skin tag evaluated by either fine needle aspirate or biopsy. In dogs when it comes to skin tumors there is about a 50% chance of being malignant. Boxers unfortunately are predisposed to many of the malignant type tumors. I do not want to scare you; most likely it is a benign skin tag, but, the only way to know exactly is evaluating the cells under the microscope.
If you have additional questions I will be glad to answer them.