It is difficult to tell from that photo if it is on the inside of it's left leg or the outside of her right leg but if it's the inside left leg, it is most likely a spur bud. Hens can get spurs too although less common and sometimes hens/pullets will just develop a spur bud that does not grow into a spur...... or depending on the age of your birds, it could be a young cockerel with a spur bud that will eventually develop into a spur. Hope that solves your query.