Hello, if it is truely a fatty tumor you will probably not have to worry about it unless it grows too much. Alot of times fatty tumors can be left alone unless they start to get bigger and bother the pet. Was a fine needle aspirate done on the tumor to make sure that is was just a fatty tumor? I would just keep an eye on things and make sure it does not get any bigger. If it does you should probaby consider having it removed so it does not interfere with anything else. Here are a few links to information on fatty tumors:
I hope this helps some but if you need anything else be sure to let me know.