Question
Do Fantails hibernate along with regular goldfish?  Do I need to keep them in an indoor aquarium for the winter?

Answer
The fantails go dormant just like other goldfish.  Sometimes fish or other animals pick at their tails while they're barely moving.  

I answered your similar question about black moors first.  The same is true for fantails although they are a little more hardy.  I had a few live for about 5 years in my pond but they too didn't live as long as the single-finned goldfish (commons, comets, and shubunkins).  I wanted more fantails in my pond so I bought three a few years back but then changed my mind. Those boys now live in a 65 gallon tank indoors where I know they're safe.  In my case, it's mostly predators getting the fancy goldfish and not necessarily the freezing weather.