Phantom limbs (sometimes referred to as supernumerary phantom limbs) are limbs that a person can feel that aren't physically there. These limbs are often compared to amputee phantom limbs, the only difference being that the phantom limbs (that otherkin feel) would not normally exist on a human.
Phantom limbs often correspond to a person's kintype. The limbs can be as simple as a pair of animal ears and a tail or wings, or as complex as the whole body of an animal or creature. They often go along with a phantom shift.
They are often incorrectly called "astral limbs", generally and Tumblr, and this is not correct. Astral shifts and astral experiences are very different from phantom shifts and experiences, and not all otherkin believe in an astral. The term phantom limb is not amputee exclusive, and phantom limbs are experienced by those who have experienced a stroke, those who are transgender, and sometimes by a rubber hand trick.