I'm sorry, that little one is deformed. It happens sometimes, just a part of nature that it happens. Although if it happens a lot, you should try not to breed any more from those adult fish, since it can also be a result of inbreeding and poor genetics. Although since livebearers store sperm, that's easier said than done. Means more that you shouldn't deliberately breed them more, and should cull the deformed and weak ones, so they don't continue passing on poor genetics.
You need to sex the fry and separate them into male and female batches as early as possible, to prevent this one from breeding, if it does survive. I've had a couple of guppies that have survived worse deformities, so she might well survive and live a pretty normal life, but you must try to avoid her being in a tank with platies of the opposite sex so she doesn't pass on the deformed genes, or risk the higher chances of dying while producing fry.
Since livebearers produce a lot of fry though, it doesn't always mean the genetics are bad, sometimes it just happens. When you have a lot of fry, occasionally something goes wrong in development or soon after they're born. My guppies have produced well over 300 fry in the past year, but only two with major spinal deformities that survived. One died fairly young, the other will live out her life with me. Sorry, not the best photo, but she has a very kinked spine that goes up and down and up again where it shouldn't, but she's over a year old and manages just fine! I didn't manage to separate the sexes early enough in this case, so she looks gravid, but she's never actually birthed fry. Fortunately, since I don't think she'd survive a birth and I wouldn't want to cull the fry, but couldn't pass them on to a store either. Best to prevent any potential breeding by separating the sexes as soon as possible. Lots of tutorials online and youtube for how to sex young fry!
