One of the softest, most luxurious fibers is pashmina wool. Pashmina comes from Himalayan goats, Capra hircus, that live in high altitudes and extreme cold. Their wool is only 12-20 microns thick. The goats are sheared once a year for their fiber.
Pashm is Persian for "wool". Most pashmina yarns sold on the market are a blend of pashmina wool and silk or other fibers. Many yarns available are lovely, such as madelinetosh Tosh Pashmina Yarn but are as little as 10% pashmina. 100% pashmina is very expensive around $100 per 100 grams. If you do buy the wool or a scarf or shawl, be sure to check the actual fiber content so that you know what you are getting.
Cashmere and pashmina are sometimes used interchangeably for the same fiber. While cashmere comes from goats, cashmere is a type, not a breed.