I'm throwing in a rummynose vote. I'm looking forward to seeing what it grows up to be.
Tangentially, something weird like this happened to me a few weeks ago. Bought three platys, from a platy tank, and I had a mystery fry after I added them to my tank at home. Platy fry, obv, right? Not so much.
As the little fish grew up, it started looking like a danio. And now, two months later, it's clearly a rainbow of some sort -- two dorsal fins. I don't have room to get him any friends, so once he's large enough for me to figure out a species, he's going back to the LFS.