Looks like an ornamental comet-tail goldfish to me, just not an orange-red one. They can be white, orange, red, brown etc after all. The well developed long fins suggest it is not even a cross-breed - ie the parents were comet tailed goldfish.
You can also get comet-tailed koi - which have more exuberant finnage than their goldfish counterparts. Maybe a hybrid with those, if theres minute barbels that would prove it.