Transmigration of the soul is similar and foreign in some ways to the philosophy of reincarnation. The idea of transmigration of soul comes from the ancient Greeks. In Transmigration after death, the soul or, shade of a living human,  loses all past memories of their previous life and then transmigrates into another form and is reborn. It was thought the soul had been, and always would be, eternal. It had no beginning or end.  Is the Greek philosophy of natural transmigration connected with a belief that the actions of the soul in one life determines the future existence.  Almost all religious persuasions (from Wiccan, to Hindu, to Christian) believe in transmigration of the soul in some form.

But there is another form of transmigration of the soul found in myths of almost every culture, from the frozen wastes of the north, though the islands, rain forests, to the volcanic blasted lands of the south.  It is the myth of the Eaters Of Souls.  These creatures from whence unknown, may have walked amongst humans from our earliest times.  Some mythologies describe the Eaters of Souls (also known as Nomads) as consuming the souls of man for sustenance and survival.  (Some believe this to be the real origin of the Vampire mythology).  But other Eaters of Souls are believed to Polythropic (also incorrectly called Polycanthropic), meaning that they can take on the shapes of the host of souls they consume.  Thus are they Amalgamorphic in nature.  These Polythropic (or Amalgamorphs), represent one of our greatest instinctual fears:  the loss of self, and the amalgamation of our soul in evil.

Amalgamorphs have been among us since the beginning of recorded time.  Many cultures have recorded creatures and humans that could transform into other creatures or people.  There are countless hybrid creatures also, possessing attributes of one animal and those of another.

In Popular Culture:

Bidirectional Amalgamorphism, although not directly referred to as such, has been used frequently to the point of being an overblown cliché in the sense of people "switching bodies," in which the identities of two or more characters transmigrate to each others bodies. This concept has been used many times in various films, most obviously Vice Versa and Freaky Friday, and in the popular television show Buffy the Vampire Slayer.  In the anime Angel Tales twelve girls were sent to live with and watch over a young man as guardian angels. All twelve were former pets of his who died and transmigrated.  It has been used as preincarnation in the television series Quantum Leap, in which the protagonist Sam Beckett would be reborn in the psychic aura of those in the past. He doesn't switch bodies psychically or mentally, but takes on their appearance through a psychic aura, while they are reincarnated as him in the future.  The concept also appears in the first and third parts of the Silent Hill series. The spirit of Alessa Gillespie transmigrated into the daughter of Harry Mason, Cheryl, after Alessa was burned to death in a cultist ritual. In the third game, the main protagonist, Heather, is actually another transmigration of Alessa's soul.

Fictional Amalgamorphs:

John Carpenter´s The Thing...

The Thing is a 1982 science fiction film directed by John Carpenter, written by Bill Lancaster and starring Kurt Russell is one of the few true movies about an Amalgamorph.  Ostensibly a remake of the 1951 Christian Nyby film The Thing from Another World, Carpenter’s film is a more faithful adaptation of the novella "Who Goes There?" by John W. Campbell, Jr., which inspired the 1951 film. The film is about a shape-shifting alien that is revived after being frozen in ice. The alien infiltrates a scientific research station in the Antarctic and kills a Norwegian research team. A nearby American research team investigates the incident and is in turn attacked by the alien.  The creature is able to assume the shape, and the very soul of the people it consumes.  In the end, none know who is a real human being, and who is the Thing.

Dean Koontz´s Phantoms...

In Dean Koontz´s story of the Phantoms: the creature's (known as the "Ancient Enemy") abilities and form greatly resemble John Carpenter's version of The Thing, the movie monster The Blob, the creature in the short story "Slime" by Joseph Payne Brennan and the "Shoggoths" envisioned in the works of H. P. Lovecraft.

The story's survivors discover that the creature consumes other life forms as sustenance, and is able to perfectly mimic any creature it consumes. It can create small "probes" or "phantoms," imitating consumed life forms, to go forth and hunt more prey, obeying the orders of its "hive mind." In addition, the creature absorbs the mental capacity and memories of those it consumes, so its mind grows more powerful, intelligent and self-aware over time. (In fact, the creature initiates contact with the survivors and personally requests the story character "Flyte" to come to the town, referring to him as its "biographer.") Besides being able to mimic real animals and people, the creature can also form phantoms based on mental images from its victims; it takes sadistic delight in creating phantoms in the shape of religious demons and monsters to terrorize its victims before killing.

The amalgamorph or shapeshifting Ancient Enemy can contort its body into the smallest of spaces (through keyholes, air ducts, etc.), which is how it managed to kill and eat people who locked themselves inside rooms and cars. It also possesses extremely keen sensory organs and extremely fast speed, which explains why even the animals around town didn't escape its predation. Conventional weapons and firearms cause no damage to its gelatinous morphic body. It feeds by completely enveloping its victims (hence the universal bruising seen depicted in the Phantoms movie) and then secreting a digestive enzyme, thus leaving no biotic traces of its victims. Toward the end of the novel the monster declares that it considers all other life forms, including humans, to be intellectually inferior to itself, nothing more than cattle to be consumed when it is hungry.

The creature discloses to Flyte and the other survivors that it has come to think of itself as Satan, due to thoughts acquired from the minds of past humans it consumed (notably a devil worshiper). The absorption of these thoughts changed the creature over time from merely predatory to overtly sadistic; it engages in a number of intentional, non-predatory evils.

H. P. Lovecraft's Shoggoths...

“It was a terrible, indescribable thing vaster than any subway train – a shapeless congerie of protoplasmic bubbles, faintly self-luminous, and with myriads of temporary eyes forming and un-forming as pustules of greenish light all over the tunnel-filling front that bore down upon us, crushing the frantic penguins and slithering over the glistening floor that it and its kind had swept so evilly free of all litter. ”  — H. P. Lovecraft, At The Mountains of Madness

The definitive description of shoggoths comes from the above-quoted story. In it, Lovecraft writes them as massive amoeba-like creatures looking like they're made out of tar, with multiple eyes "floating" on the surface. They are described as "protoplasmic", lacking any default body shape and instead being able to form limbs and organs at will. The size of an average shoggoth measured 15 feet across when a sphere, though the story mentions ones of much greater size.

Although intelligent to some degree, Mythos media most commonly shows them dealing with problems using their great size and strength. For instance, the original one mentioned in The Mountains of Madness simply rolled over and crushed giant albino penguins that were in the way as it pursued the characters.

The shoggoths are considered one of the more terrible things present in the Mythos. The character of the Mad Arab, Abdul Alhazred, found the mere idea of their existence on Earth terrifying.

Douglas Preston & Lincoln Child's Relic...

Relic is a monster on the loose in New York City's American Museum of Natural History provides the hook for this high-concept, high-energy thriller about another type of Amalgamorph. A statue of the mad god Mbwun, a monstrous mix of man and reptile, was discovered by a Museum expedition to South America in 1987. Now, it is about to become part of the new Superstition Exhibition at the museum ("New York Museum of Natural History" in the book, and the "Field Museum" in Chicago in the movie). In this story, the amalgamorphic creature, known as "Mbwun" - an Amzonian mythic beast - derived its physical changes from the DNA of its victims and a biological agent that promotes the transformation.  Less horror then action-adventure, the narrative builds to a superbly exciting climax, and then offers a final twist to boot. With its close-up view of museum life and politics, plausible scientific background, sharply drawn characters and a plot line that's blissfully free of gratuitous romance, this well-crafted novel offers first-rate thrills and chills, and an opportunity to view a mythic monster from a plausible scientific perspective.

Mythical Amalgamorphs:

Werewolves, also known as lycanthropes or wolfmen, are mythological humans with the ability to shapeshift into wolves or wolf-like creatures, either purposely, being bitten by another werewolf or after being placed under a curse. The medieval chronicler Gervase of Tilbury associated the transformation with the appearance of the full moon; however, there is evidence that the association existed among the ancient Greeks, appearing in the writings of Petronius. This concept was rarely associated with the werewolf until the idea was picked up by Gervase. Shape-shifters similar to werewolves are common in tales from all over the world, though most of them involve animal forms other than wolves.  However, many are not true Amalgamorphs, they simply "Change" there appearance and attributes to those of the animal they become. 

But there are examples in the Werewolf legends of amalgamorphism.  These stories involve the original of the werewolf, as a man eaten by a wolf, with the wolf consuming his flesh and soul, then becoming human again for brief intervals, then returning the the form of the wolf again, and again.

Werewolves are a frequent subject of modern fictional books and films, although fictional werewolves have been attributed traits distinct from those of original folklore, most notably the vulnerability to silver bullets.

How To Kill An Amalgamorph:

From a practical standpoint, most Amalgamorphs is fiction and mythology are living creatures that are subject to at least some the laws of physics.  Thus substantial disruption of biological functions results in death.  However, disrupting those functions may prove challenging, if the creature assumes body shapes and defenses that make it resistive to damage.

In mythology, and number of methods evolved to deal with the Amalgamorphic Creatures.  In the case of were-folk, this involved silver bullets.  In the case of vampiric creatures, this was a wooden stake in the heart.  Cutting of the head had effect with some creatures, but other were able to regrow it.  Use of explosives, in many myths, simply results in the "spreading" or "dividing" the creature into many.  However, the most accepted method in mythology is death by fire, and reducing it to ash.  This destroys the creature and prevents further misadventure and resurrection in other form.