Since there seems to be a head on it I would say that is an abscess. You can try sitting him/her is warm water a few times a day or warm compress to get it to open up. You will then need to flush it out a few times a day. Do not put any ointment on it - it need to heal from the inside out. You may want to give you rattie some pain meds.
Not an abscess then. It would have popped by now. So either a cyst or tumour. If it has been present with no change for a year, I would say it's safe to keep watching it.
Where on the piggy is it? If it is on the stomach, it could be a hernia too.
thats what I would guess. I will take him to the vet when I have the extra money because I'm sure removal of whatever will be really expensive. He is acting A-OK though. Still his happy little self. Located between his hip and stomach