that fish has had a problem with development. probably a dietary/nutrient defficiency. a very big ask for it to come good in way of posture. i come across koi fish like this sometimes. some say its a lack of vitamin c etc. some species of fish you can get a whole heap of bent backs/bad heads etc and they even blame it on the nutrition going into the parent as well as rearing conditions. take care of it and it could still live a long life. its amazing but ive seen goldfish with their half their face and all their lips/jaw chewed off by water rats and they still can live if they can get food.