It may be a prolapse of sorts. I have treated a prolapse with epsom salt baths and fasting. In my experience (maybe 3-4 fish, all smaller livebearers over the years), most times the prolapse reabsorbed into the body over a few days.
I have heard they may reabsorb on their own, so I have no idea if the epsom salt was directly responsible for the success.
In your case, I don't think I'd risk the stress of netting an 18 year old bigger fish multiple times a day. I don't know what else is in the tank, but you may consider adding epsom to the tank for a few days. If you do, I'd do a low dose and be aware it may raise your GH some.