Any person or entity can create a currency. They can even make it more valuable than their national currency.
If a person creates a product or service that others need, they can create a new currency as the only accepted form of payment for that product or service.
As the product or service becomes increasingly in demand, it will generate demand for the new currency. People will exchange other currencies for the new currency. Some people may use the newly created form of payment among themselves to the point where it is preferred - thus, how the US dollar retains its value. Otherwise, the US Dollar would be worthless.
In the future, a large corporation that controls many products and services may develop a currency and use it to control people by denying supply to the people that do not conform to the corporation's values.