I'm a yoga instructor! Yoga is a group of mental, physical and spiritual practices which started in ancient India!
How can we classify yoga? a spiritual exercises?
It definitely started as a spiritual exercise, but as of the 1980s, it became a popular physical exercise practice in the Western world.
Does each of the oriental religion like Hinduism, Buddhism and others  have their own form of Yoga?
Yoga
Yoga (; ; ) is a group of physical, mental, and spiritual practices or disciplines which originated in ancient India. Yoga is one of the six orthodox schools of Hindu philosophical traditions. There is a broad variety of yoga schools, practices, and goals in Hinduism, Buddhism, and Jainism. The term "yoga" in the Western world often denotes a modern form of Hatha yoga, yoga as exercise, consisting largely of the postures called asanas.
The origins of yoga have been speculated to date back
Indian traditions has been more than physical exercise; it has a meditative and spiritual core. Outside India, "yoga" has come to mean a form of posture-based fitness regimen, a stress-relief and relaxation technique. Modern yoga has adopted innovations from Western gymnastics and other practices.
Some versions of modern yoga contain reworkings of the ancient spiritual tradition, and practices vary from wholly secular, for exercise and relaxation, through to undoubtedly spiritual, whether in traditions like Sivananda Yoga or in personal rituals. Modern yoga