whats this

[Wandering_Angler]

I think actually it is a comet goldfish mixed with koi. A fantail has the tail fin split into three where as a comet just has the usual two forks but elongated. Niether fantail or comet can get this big and comets are usually either all red/gold or red/gold mixed with white (in which case that are known as sarasa comets). Mixed with a koi this size and colour would be likely together with the fin array displayed by this fish. Hope this helps :)