The original-yoga is not yoga
Originally, the yoga is either a physical exercise, nor a discipline made by postures of the body. The word yoga means: union since the samnyasin monk; Adi Shankara, around the eighth century BC. Before, this word meant freedom or liberation. This freedom is the one that a person finds when it is not any more the prisoner of the mind's fluctuations and its residues. The Union is rather an assembly in a point: that of the Consciousness around its core. The path is a practical spirituality which gives tools to gather your Consciousness.
There is only one yoga, that is only one freedom, union but various ways which set to allow you knowing them. The freedom, the union and the methods to arrive bear the same name, the yoga and methods, there are plenty of them. Just enter the word yoga in a search engine to have almost an endless list : new "yoga" are created regularly.
The path of freedom is not a yoga requiering physical postures nor to recite mantras. We name it original-yoga because we argue that it is at the origin of every yoga with spiritual aim, as the jnana, the bhakti, karma* and the raja-yoga of the Yogasûtra. At the time of the writing of the book, the writers practised the yoga which belonged to the philosophic school Sàmkhya resting on Shruti or revealed Knowledge which is, today, the path of freedom or the original-yoga.
There is a version of Yogasûtra which takes into account this historic fact and the spiritual and linguistic context of the time: the treatise of the original-yoga.
*Krishna taught this yoga to Arjuna. Yoga quoting in Bhagavad-Gita formed only one, at the time, as the path today.