I've had zebra, leopard and pearl danios; espei, red-tailed, fire and scissortail rasboras; and white cloud mountain minnows all show up in other tanks simply because I transferred java moss from one tank to another. One time was a double surprise, because I had zebra danios and scissortail rasboras in the same bunch. There might have been another species or two in there for all I know, because numerous times I've had fry that I've noticed but didn't survive the transfer. I've never had total mystery fry that showed up in a batch of plants from a friend or a store, though.
Honestly, that guy looks like a danio to me, but obviously I'm wrong, so now I'm stumped.
WYite