This is a first for me.....?

Some mutation seems to be a regular part of fish reproduction. Most mutations may inconvenience the fish or make it stand out of the crowd. They don't usually get to reproduce.

Once in a while a mutation provides the fishes with a survival advantage and those may continue on. That your other platys are full sized and robust excludes the possibility that you were giving them less than excellent care and they were stunted. So there must be a least another explanation for that male.

I'm sure that you are aware that our hobby swordtails, platys and even variatus have been extensively crossed with one another. Indeed a lot of that was done from the 1930s because some crosses can be depended upon to produce a melanoma and those fish are used in cancer research. The successor to that lab is the Xiphophorus Genetic Stock Center, now at Texas State University, San Marcos. In addition to taking care of all sorts of Xiphophorus strains, they have a number of research resources such as the XiphBase - A Genomic Toolset for Xiphophorus Researchers.

Among their other projects is their http://www.xiphophorus.txstate.edu/stockcenter/ser...

They make strains available to researchers all over the world and if they have extra stock, as of X. montezumae, they can make those strains available to hobbyists too.

Attendees of the ALA (American Livebearer Association for someone looking one) convention in Texas visited that center a few years ago. Some bought fish.

The ancestral hobby swordtail species seems to have been Xiphoporus helleri, but according to the ALA and other livebearer forums, other swordtail species may have been snuck in too. I'm wondering if your male platy (there are red wild swordtails (see the Baensch Atlases, especially their Photo Atlas) perhaps was expressing some of its swordtail ancestry.

About 15 years ago in the journal Copeia (in one of the last bound copies before everything went electronic) there was a field study exploring why X. helleri threw both large males and smaller males, who seldom had a chance to mate unless they assumed the role of "sneaker males." The authors suggested that both sized males could help the species survive.

The larger males that took so long to mature (probably giving rise to hobby accounts of females turning into males) are actually able to sequester a small territory of his own. They are attractive to large females, perhaps programmed to appreciate large males. The advantage to her is that she spends less energy (food) avoiding pesky smaller males. That allows her & him to eat more, have large drops (in size, numbers & vitality) and spend less time foraging in more open, predator rich areas.

However there was also a survival benefit in having the sneaker males. In times when predation is heavy the fact that every few 100 tries a small male actually inseminates a female means that younger swordtails may be dropping fry even as most larger ones (targeted first by some predators) are being picked off.

Similar situations are describe by Anne E. Houde in Sex, Color, and Mate Choice in Guppies. In predator free ares the best colored males and strong courters are selected by more females. (The best dancers get the girls.) But in areas of heavy predation females avoid pike cichlid attracting displays. They are more likely to tolerate sneak plain colored spawners under those circumstances.

Enjoyed your question. Watching your fish that closely is one of the reasons you are still keeping fish, sharing your insights and enjoying the hobby. You & I probably both bought some of our first fish at a Woolworth's. :)

I bred swordtails for a while, and stopped because I couldn't handle them anymore. However, I did end up with a couple of functional dwarfs as I don't have the heart to cull fry. He may be a healthy dwarf, but remember when there are external abnormalities, there are often internal abnormalities as well, that may not show... So might not be a good idea to produce fry from this fish, unless it seems fully healthy otherwise. However if it is healthy, you may have created a dwarf platy hehe.

I've also had swordtails that grew into adults, and never got any color what so ever, and remained clear-tan. Han't seen two heads yet, but I've seen two headed and two tailed trout sac fry, and black sea bass without operculums.

mutations happen all the time, we don't even notice many because the fry aren't viable and die

it's considered bad to keep and breed mutations usually because it screws up the genetics of the fish and can get back into larger fish populations and spread..