I think you got solid advice. Swellings on the caudal peduncle don't occur out of the blue or probably ever, unless there is an obvious and significant injury and there is evidence of the injury or of an attack.
It is almost 100% sure a kink in the skeletal structure and will never go away. Deformed fish like this appear to live fine, but defects and mutations tend not to occur alone but in groups, and thus, who knows what other latent defects this RTC has, again if inborn (if an accident, this logic wouldn't apply)... The kink has nothing to do with the appetite swings - these usually have the underlying reasons as stress, illness, seasons, natural cycles of fish biology.
You spoke of discoloration too in your OP. What discoloration?