It could be a lot of different things and from this description it is impossible to say. Three things that come to mind are: a congenital defect (i.e., born crippled), an injury, or Mareks disease. I had a crippled turkey poult this year. At first she was able to get around well enough but it got worse the bigger she grew (and heavier she got) and at 12 weeks I decided it was kindest to cull her, as by that stage she was no longer able to get to food and water very well and would not have been able to run and hide from a predator. Euthanizing her seemed more humane than watching her slowly starve to death.
