Wankan (Kings Crown) is also known as Ohkan (which means the same thing in Japanese). The history of all the Shotokan kata is hazy at best and this applies doubly for Wankan as not much at all is known about its origins.
In some styles there is another kata practiced that shares the name but has totally different moves to the Wankan of Shotokan, which may suggest that they are both only a portion of a longer, lost original kata. If this is the case, then we can say that it may have been brought to Okinawa from China and taught to Sokon Bushi Matsumura, but this is mostly speculation.
The name is thought to be taken from the first three moves, which when viewed from the front, resemble a crown. The first two moves resemble the two side diadems and the third move creates the centre piece of the crown. If this is more than mere coincidence then it is very interesting as Okinawan kings are not recorded as wearing metal crowns as medieval kings did in Europe.