My rat Nessie has a big lump near her tail. Pet's info: Small Animal | Rat | Female | unspayed | 1 year and 4 months old

Thanks for using Petco Pet Education Center, formerly Petcoach! This may be a benign or malignant growth. This could even be an infection. This cannot be completely assessed just by looking at pictures and usually requires sampling such as fine needle aspiration to determine its origin. Sometimes a biopsy is needed for a definitive diagnosis. I recommend that you have her evaluated by your veterinarian for further assessment and treatment. I am really hoping for something benign. Good luck and have a good day!

There are a couple of possibilities for what the lump is on Muffin. It could be a mammary tumor, or and abscess. The only way you will be able to definitively tell which it is is to take her to a veterinarian for a fine needle aspirate, where the vet will stick a needle into the lump and look at the collected cells under the microscope. Either way she will likely need surgery to remove it, and there is a chance that it is too big, or in a risky location for surgery. If she is otherwise well in herself then you can continue to monitor her and look for signs of her being uncomfortable. However if she stops eating, drinking, acts lethargic or starts having trouble moving due to the lump then she needs to see a veterinarian as it has become a quality of life issue and she may need to be euthanized.