From D.T.Suzuki (Manual of Zen Buddhism) ..
The author of these "Ten Oxherding Pictures" is said to be a Zen master of the Sung Dynasty known as Kaku-an Shi-en (Kuo-an Shih-yuan) belonging to the Rinzai school. He is also the author of the poems and introductory words attached to the pictures. He was not however the first who attempted to illustrate by means of pictures stages of Zen discipline, for in his general preface to the pictures he refers to another Zen master called Seikyo (Ching-chu), probably a contemporary of his, who made use of the ox to explain his Zen teaching. But in Seikyo's case the gradual development of the Zen life was indicated by a progressive whitening of the animal, ending in the disappearance of the whole being. There were in this only five pictures, instead of ten as by Kaku-an. Kaku-an thought this was somewhat misleading because of an empty circle being made the goal of Zen discipline. Some might take mere emptiness as all important and final. Hence his improvement resulting in the "Ten Oxherding Pictures" as we have them now.
According to a commentator of Kaku-an's Pictures, there is another series of the Oxherding Pictures by a Zen master called jitoku Ki (Tzu-te Hui), who apparently knew of the existence of the Five Pictures by Seikyo, for jitoku's are six in number. The last one, No. 6, goes beyond the stage of absolute emptiness where Seikyo's end: the poem reads:
"Even beyond the ultimate limits there extends a passageway,
Whereby he comes back among the six realms of existence;
Every worldly affair is a Buddhist work,
And wherever he goes he finds his home air;
Like a gem he stands out even in the mud,
Like pure gold he shines even in the furnace;
Along the endless road [of birth and death] he walks sufficient unto himself,
In whatever associations he is found he moves leisurely unattached."