Hags are invariably female, they appear scrawny, but their strength is uncanny and they can crush a goblin with one hand. A hag can move swiftly and leap over unexpected distances. She has iron-like claws to rake and rend her victims.
Night hags are members of Sluagh. They are physically stronger than Sidhe, harder to kill and heal well. Nerys is confident she would survive a bullet through her face.
Based on the threat Nerys used against Meredith in A Kiss of Shadows, it seems possible one can become a night-hag by being cursed.