There's atypical anorexia, OSFED that can happen at any weight, and its characteristics are rapid weight loss or weight fluctuations as well as disordered  behaviour,  but the dangerously low weight is a criteria you need to have in order to be diagnosed with anorexia nervosa. But a lot of fats keep saying "it's a mental illness!  It happens at any weight" which is just not the case.