A parasitic twin (also known as an asymmetrical or unequal conjoined twin) is the result of the processes that produce vanishing twins and conjoined twins, and may represent a continuum between the two. Parasitic twins occur when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of the other. Unlike conjoined twins, one ceases development during gestation and is vestigial to a mostly fully-formed, otherwise healthy individual twin. The undeveloped twin is defined as parasitic, rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus.
- Conjoined-parasitic twins joined at the head are described as craniopagus or cephalopagus, and occipitalis if joined in the occipital region or parietalis if joined in the parietal region.
- The Twin reversed arterial perfusion or TRAP sequence, results in an acardiac twin, a parasitic twin that fails to develop a head, arms and a heart. The parasitic twin, little more than a torso with or without legs, receives its blood supply from the host twin by means of an umbilical cord-like structure, much like a fetus in fetu, except the acardiac twin is outside the host twin's body. Because it is pumping blood for both itself and its acardiac twin, this causes extreme stress on the normal fetus's heart. This twinning condition usually occurs very early in pregnancy.
- Fetus in fetu sometimes is interpreted as a special case of parasitic twin, but may be a distinct entity or a special case of teratoma.
- Pinoko (ピノコ), from the Black Jack manga, a little girl constructed by him who is actually a Teratoma, a rare type of parasitic twin, living in one of Black Jack's patients' bodies for eighteen years until he extracted her and gave her a real body, a plastic exo-skeleton.
- Kuato, from Total Recall, a mutant with psychic powers on a futuristic Mars.
- In Billy Cowie’s novel Passenger  the main protagonist Milan Kotzia spends the entire book learning how to communicate with his parasitic sister Roma living inside himself. 
- Basket Case, a horror film in which a man carries his recently-separated, deformed parasitic twin around in a basket.
- Brothers of the Head, a novel (and now a film) involving conjoined twin brothers with a parasitic triplet.
- The Dark Half, a novel by Stephen King about an author with a pseudonym that begins physically manifesting himself and is ultimately revealed to be a parasitic twin that was absorbed by the author in utero.
- South Park - Conjoined Fetus Lady, TV episode, season 2, episode 5. It is discovered that the school nurse has a conjoined (parasitic) twin attached to the side of her head and the community make typically misguided attempts to make her feel welcome.
- The Hills Have Eyes. The character Papa Jupiter has one.
- ^ Aquino DB, Timmons C, Burns D, Lowichik A (1997). "Craniopagus parasiticus: a case illustrating its relationship to craniopagus conjoined twinning". Pediatric Pathology and Laboratory Medicine 17 (6): 939-44. PMID 9353833.
- ^ Acardiac Twin or TRAP Sequence. University of California, San Francisco (2007-04-26). Retrieved on 2007-05-30.
- ^ Cowie B (2007) Passenger. Brighton ISBN 9780955400407
- ^ http://books.guardian.co.uk/reviews/roundupstory/0,,2025326,00.html