Cornish cross meat bird. That, unfortunately, is quite bad.
Not at all meaning to be cruel - but these birds are literally not bred to survive. They're bred to grow as wide as they are tall in a very short amount of time, and when allowed, or just lucky enough to physically make it longer than that, their lives usually suck. Their legs don't work right, they're too heavy to properly run, fly, forage - and not to mention their organs often straight up fail. Their little hearts and lungs were just not meant to keep up with all that body mass.
If humanely culling them is not a realistic option for you (understandable, I doubt I would either if I somehow came in possession of one of these), you can do what you can to make the best of their situation. Healthy diets, managing their exercise (like, just getting them off their fat bellies, lol.) And it could work out. Some do survive into adulthood and can be quite happy.