it looks like a Rainbow fish or perhaps some killI that got into the water at the pond where they raise the local feeders by you (ive gotten some nice america native fish that have been shiped out with feeder ghost shrimp )
what Scotty said. dither fish usually refer to a faster more energetic fish that makes bigger more timid fish come out of hiding (ex. "if it is safe for them it must be safe for me") or gets the attention of more aggressive fish so that they will not attach other tank mates. what you have could be a common dither fish, but that is not actually the name of that particular fish.