It's an endlers livebearer (Poecilia wingei) It is like the guppy's 'cousin' and it is rare, the one you have is a male, females are a grey/ gold colour. It will cross breed with guppies so try not to keep them with guppies to save hybridisation.
It may be an orange spot endler but hard to tell until full colour is shown. All endlers are rare.
I got a free "rare" fishy ? COOL