Polycephaly
Polycephaly is the condition of having more than one head.  By far the most common use is in relation to the anatomical head, though the word has also been used for other meanings of "head".  The term is derived from the stems poly- meaning 'many' and kephal- meaning "head", and encompasses bicephaly and dicephaly (both referring to two-headedness).  A variation is an animal born with two faces on a single head, a condition known as diprosopus.  In medical terms these are all congenital cephalic disorders.
There are many occurrences of multi-headed animals, in real life as well as in mythology.  In heraldry and vexillology, the double-headed eagle is a common symbol, though no such animal is known to have ever existed.
Bicephalic or tricephalic animals are the only type of multi-headed creatures seen in the real world and form by the same process as conjoined twins: the zygote begins to split but fails to completely separate.  One extreme example of this is the condition of craniopagus parasiticus, whereby a fully developed body has a parasitic twin head joined at the skull.

# Occurrences
Two-headed people and animals, though rare, have long been known to exist and documented.  The "Scottish brothers" were conjoined twins, allegedly dicephalic, born 1460 (dates vary).  (Human conjoined twins, not all of the dicephalic type, have been documented since the year 945.)

## Novelty and study
Polycephalic animals often make local news headlines when found.  The most commonly observed two-headed animals are tortoises and snakes.  Other species with known two-headed occurrences include cattle, sheep, pigs, cats, dogs, and fish.  In 1894, a two-headed partridge was reported in Boston, Massachusetts. It was notable as a dicephalic animal for surviving into adulthood with two perfect heads.  Scientists have published in modern journals about dissecting such animals since at least the 1930s.  A 1929 paper studied the anatomy of a two-headed kitten.
Polycephalic animals, due to their rarity, are a subject of novelty.  "We", a two-headed albino rat snake born in captivity in 2000, was scheduled to be auctioned on eBay with an expected price tag of $150,000 (87,000), though their policy of not trading in live animals prevented the sale. On 2006-10-31, the World Aquarium announced that "We" was adopted by Nutra Pharma Corporation, a biotechnology company developing treatments using modified cobra venom and cobratoxin. "We" died of natural causes at age eight in June 2007.
Two-headed farm animals sometimes travel with animal side shows to county fairs.  Many museums of natural history contain preserved two-headed animals.  The Museum of Lausanne in Lausanne, Switzerland, and the Ripley's Believe It Or Not! museum in Gatlinburg, Tennessee, have collections of preserved two-headed animals.

## Anatomy and fitness
Each head of a polycephalic animal has its own brain, and they somehow share control of the organs and limbs, though the specific structure of the connections varies.  Animals often move in a disoriented and dizzy fashion, with the brains "arguing" with each other; some animals simply zig-zag without getting anywhere  Snake heads may attack and even attempt to swallow each other.  Thus, polycephalic animals survive poorly in the wild compared to normal monocephalic animals.
Most two-headed snakes only live for a few months, though some have been reported to live a full life and even reproduced with the offspring born normal.   A two-headed black rat snake with separate throats and stomachs survived for 20 years.  "We", the two-headed albino rat snake, survived in captivity for 8 years before passing away.   There is some speculation that the inbreeding of snakes in captivity increases the chances of a two-headed birth.
A famous successful modern case is that of the human dicephalic twins Abigail and Brittany Hensel, born in 1990.  The twins have two heads, two hearts, four lungs, and two spinal cords, and share the other organs.  Each twin controls the limbs on her "side", and with coordination can walk, run, and play piano.

## One or two animals?
It is difficult to draw the line between what is considered "one animal with two heads" or "two animals that share a body."  With humans, dicephalic conjoined twins such as Abigail and Brittany Hensel are considered "twins", i.e. two individuals .This makes sense as there is a range of conjoinedness, and non-dicephalic conjoined twins may be barely-conjoined and separable by surgery, as is the case with Chang and Eng Bunker.  Although the Hensel twins only have one pair of arms and legs total, each head controls one "side" of the body's limbs.  On the other hand, Syafitri, born 2006 in Indonesia, was given one name by her parents because she only had one heart.  With other animals, polycephaly is usually described as "one animal with two heads." One of the heads, especially in three-headed animals, may be poorly developed and malformed, and not "participate" much.

# Oldest occurrence
The February 22, 2007 isue of the journal Biology Letters detailed the discovery of a 120 million-year-old fossil of a 2-headed Hyphalosaurus lingyuanensis, marking the earliest known occurrence of axial bifurcation.

# List of recent occurrences

## Humans
Dicephalic conjoined twins (dicephalus dipus):
- In 1990, Abigail and Brittany Hensel were born in Minnesota, United States
- In 2003, Sohna and Mohna were born in India
- In 2004, Rebeca Martinez was born in the Dominican Republic with an extra head but died 7 hours after surgery.
- In 2005, Egyptian Naglaa Mohamed gave birth to Manar who had the head and a remnant of a torso of another child attached. Surgery was performed for separation.   Naglaa Mohamed made an appearance on the Oprah Winfrey show with her surviving  child.
- In 2006, Syafitri was born in Indonesia
- In 2007, Mary Grace and Mary Divine Asis was born in the Philippines having only one heart. 

## Non-human animals

### Cats
There have been numerous reports of two-faced cats; most die soon after birth.  Reports of two-headed kittens are common, relative to other animals, because of their status as household pets.  Recent two-headed kittens include:
- On 2000-06-07, Image, a two-faced kitten, was born on and died later that year in Philadelphia, Pennsylvania.
- On 2005-03-08, Deuce, a two-faced kitten, was born in Lake City, Florida, and was euthanased shortly thereafter, having come down with pneumonia.
- On 2006-06-12, Gemini, a two-faced kitten was born in Glide, Oregon.
- On 2006-07-12, Tiger, a two-faced kitten was born in Grove City, Ohio.
Polycephalic cats in museums include:
- The Museum of Lausanne in Lausanne, Switzerland, preserves a two-headed kitten (pictured).
- The Laing Museum in the small town of Newburgh, Fife, Scotland, preserves the stuffed body of a two-headed kitten born in the 19th century on Mugdrum Island.
- The Georgia State Capitol in Atlanta, Georgia, has a full body taxidermy of a two-faced kitten.

### Cattle
- A head mount of a two-headed calf is on display in the Museum at the Georgia State Capitol Building in Atlanta, Georgia.
- A two-faced calf is preserved at the Douglas County Museum in Waterville, Washington. The calf lived for ten days after birth.
- The Ripley's Believe It or Not! museum in Gatlinburg, Tennessee, has full body taxidermy of a two-headed calf.
- The Dalton Gang Museum, located in Meade, Kansas, also displays a full body taxidermy of a two-headed calf.
- A two-headed calf mount can be found at the Old State House in Hartford, CT
- A full taxidermy of a two headed calf can be found in Melton Mowbray museum, Leicestershire, Uk
- A full taxidermy of a two headed calf can be found in the Museum of Marxell (in the Northern Black Forest in Germany). The calf was born by a local cow and died shortly after birth by natural causes.

### Pigs
- In 1998, Rudy, a two-headed pig was born in Iowa.

### Goats and sheep
- In 2006, a two-headed lamb was born in Shandong, China.
- The Ripley's Believe It Or Not! museum in Gatlinburg, Tennessee, has a mount.

### Reptiles
- A two-headed black rat snake with separate throats and stomachs survived for 20 years.
- There are several preserved two-headed snakes on display in the Museum at the Georgia State Capitol Building in Atlanta.
Two-headed turtles and tortoises are rare but not unknown.  Recent discoveries include:
- In 1999, a three-headed turtle was discovered in Tainan, Taiwan, by a villager named Lin Chi-fa.
- In 2003, a two-headed angulate tortoise was discovered in South Africa, with the only other known case in the region reported in the early 1980s.
- In  2004, Solomon and Sheba, a two-headed Mediterranean spur-thighed tortoise was born in Dorchester, England.
- In 2005, a two-headed olive ridley sea turtle was found in Costa Rica by the World Wildlife Fund.
- In 2005, a baby turtle of unknown species was also reported in Havana, Cuba, in 2005.
- In 2006, a two-headed, five-limbed soft-shell tortoise in Singapore was also featured on a local television program, and again on another program in late 2006.
- As of 2006, a living two-headed tortoise named Janus is being displayed in the Museum of Natural History of Geneva.
- There is a full body mount of a crocodile with two heads located at the Georgia State Capital on the top level.

# Mythological occurrences

## Greek mythology
Greek mythology contains a number of multi-headed creatures. Typhon, a vast grisly monster with a hundred heads and a hundred serpents issuing from his thighs, is often described as having several offspring with Echidna, a creature with the body of a serpent but the face of a beautiful woman. Their offspring account for all the major monsters of Greek mythos, including:
- The Nemean Lion  a lion often depicted with multiple heads
- Cerberus  a monstrous three-headed dog that guards the gate to Hades
- Ladon  a sometimes hundred-headed serpentlike dragon that guards the garden of the Hesperides and is overcome by Heracles
- Chimera  sometimes depicted with the heads of a goat and a lion
- The Lernaean Hydra  an ancient nameless serpent-like chthonic water beast that possessed numerous heads
- Orthrus  a two-headed dog owned by Geryon
- The Hecatonchires were giants with fifty heads and one hundred arms. The word "Hecatonchire" means "hundred arms". They were the sons of Gaia, and Uranus.
Other accounts state that some of these creatures were the offspring of Phorcys and Ceto.  Phorcys is also said to have fathered Scylla, a giant monster with six dogs' heads, which terrorises Odysseus and his crew.

## Other mythologies
- Balaur, a dragon with three, seven or twelve heads, in Romanian mythology
- Bune, a dragon with the heads of a dog, a griffin, and a man, in Christian demonology
- Janus, a two- or four-faced god in Roman mythology
- Kaliya, a multi-headed snake vanquished by Krishna in Indian mythology
- Nehebkau, a two-headed snake in Egyptian mythology
- Orochi, an eight-headed snake in Japanese mythology
- Ravana, the ten-headed King of Lanka from the Hindu smriti Ramayana
- Svantevit, four-headed god of war and divination in Slavic mythology
- Triglav (meaning "three headed") is a god or complex of gods in Slavic mythology
- Zmey Gorynych, a dragon in Slavic mythology

# Modern Fiction
Modern fiction does not like anatomical variation as much as mythology, perhaps because of the need to give specific detais, which is difficult to do if the thing one is describing is impossible. There is one obvious example: the character
- Zaphod Beeblebrox, in the Hitchhiker's Guide to the Galaxy