Homunculus

Homunculus (Latin for "Little Human") refers to the legendary concept of an artificially created human, presumably brought into existence by certain means of alchemy. Though a common concept, well known and understood by alchemists in Amestris, the idea of homunculus is regarded as no more than a farce or faraway fantasy as no individual or group of alchemists has come anywhere close to successfully creating such a being in officially recorded history. Even so, alchemists have been forbidden by the Amestrian government to attempt or research the transmutation of humans and State Alchemists have such a restriction as part of their creed.

However, off the official record, not only one, but several of these creatures have been created in arcane, sinister secrecy. In the most relevant sense of the word, "Homunculus" refers to the eight individuals who make up the primary antagonistic force of the Fullmetal Alchemist series. The Homunculi created by Father (with the sole exception of Pride) carry the mark of the Ouroboros upon their bodies and are identified as such by said mark. Other common things that some of them share are hueless hair color, and the same eye color, which is shown to be lavender in the 2003 series and mauve in the 2009 series.

2003 Anime
Because of the divergence in plot line in the 2003 anime, the origins, identities and secrets of Homunculi differ from those of the manga and second television series.

In the 2003 anime, Homunculi are said to be created each time an alchemist attempts a Human Transmutation. While the transmutation itself will result in a failure in that the person intended to be revived does not return as expected, a new existence is brought about. The result of failed Human Transmutation will be a grotesque, vaguely humanoid monstrosity retaining the genetic material, as well as some of the memory, of the resurrected. The "failed" Homunculus generally remains in this agonizing state until it dies soon after. However, newly created Homunculi who come into contact with Red Stones—much weaker versions of the Philosopher's Stone created by condensing human souls—and consume large amounts of them have their bodies reshaped into their intended forms and become exact replicas of the entities they were meant to replace.

The English dub of the anime states that Homunculi aren't failed human transmutations, but that they are simply incomplete.

Killing Homunculi in the 2003 series can be done in a variety of different ways, but the task is still a challenge due to the difficulty in dealing with such deadly and unpredictable creatures. Furthermore, the fact they are "mythical" creatures shrouds the means in mystery until Greed reveals all before dying himself. Once the means are discovered, it is often necessary to take advantage of more than one of their weaknesses in order to kill them, the most important of which is the remains of the person they were meant to be, which are not always available. (Again, Wrath is not susceptible to this weakness because his remains no longer exist. He does have other weaknesses however; he is terrified of the Gate and paralyzed by the sound of a baby's cries.)

When Homunculi die, their bodies liquefy into a red ooze which quickly dissolves into the ground, leaving no trace that they ever existed.

Trivia

 * The seven Homunculi are named after the Seven Deadly Sins.
 * The bodysuits that most of the Homunculi wear, have a slight brownish-red tint in the 2009 anime. In the 2003 anime, they had a dark green hue to them. Occasionally, in both series, they look more black, akin to their color in the manga.
 * Most of the Homunculi have red nodes connected by lines, which seem to incapacitate them and/or cancel out their powers when pierced, e.g. Lust in the 2003 anime and Greed when he is captured by Wrath in the manga and 2009 anime.The only Homunculus who do not have apparent nodes are Pride (manga), Father (in his humanoid form), Second Greed (Ling's body, manga), and possibly Wrath (manga). Wrath and Sloth in the 2003 anime also seem to possess the nodes and lines as well. However, because some of the lines and nodes are on the articles of 'clothing' that the Homunculi wear, the question stands whether there are identical lines beneath the clothes or if the clothes are connected to Homunculi physically. Some examples are Lust's 'gloves' and Greed's bodysuit.

Manga and 2009 Series

 * The permanent deaths of the Homunculi occur in almost the exact opposite order as the levels of the mountain (from bottom to top) of Purgatory in Dante's Purgatorio, the only exceptions being that Pride didn't die and Envy and Greed's places were switched (Envy dying after Gluttony and Greed dying after Wrath).
 * While a Homunculus' memories can be 'wiped' by absorbing them back into Father, they are not completely erased. Rather, the memories will be pushed down in the mind of the Homunculus, often only resurfacing during a powerful emotional event. This is seen in the case of the Second Greed, who remembers his friends after killing Bido.
 * Father's largely prideful nature and the presence of pride in every homunculus may be references to pride being the father of all sins.
 * In the manga, all of the named Homunculi die in a way related, either directly or inversely, to the sin they 'embody':
 * Lust was incinerated to death by Roy Mustang (who is a womanizer) to defend Riza Hawkeye and Alphonse Elric.
 * Greed was killed by a power-hungry Father after he discovered what he truly wanted. He also sacrifices himself to weaken Father in the final fight.
 * Gluttony was devoured by Pride.
 * Envy commits suicide out of jealousy and self-loathing toward humans. (Defeated by Riza Hawkeye and Roy Mustang together.)
 * Sloth dies from over-exerting his Philosopher's Stone during his battle against Alex and Olivier Armstrong, and Sig and Izumi Curtis.
 * Wrath was dismembered by a once-vengeful Scar, and dies a relatively peaceful death with a smile on his face.
 * Pride was spared by Edward; however, he was stripped of his power and reduced to an even weaker form of the creatures he considered inferior.
 * In an Omake, Father scolds the Mannequin Soldiers for not having any underwear on.
 * Animals seem to have the ability to recognize and detect Homunculi, most likely because they can sense the unnatural density of souls within them. Both Black Hayate  and Xiao-Mei  both able to identify Gluttony and Envy simply by being in proximity with them. (Black Hayate was even given an unofficial promotion for this skill.) Most animals become unnerved in their presence, including the horses at Briggs, several pets that lived in Central, the rats/mice that lived underground in Central, and even the vicious guardian chimera that protected Father's lair. The human chimera were also shown to become ill at ease while around them, though they could never explain just why.
 * It's possible that human-based homunculi cause less of a reaction in animals.
 * Upon first meeting Greed/Ling, Heinkel and Darius didn't realize he was a Homunculus until after he told them so.
 * Den didn't seem to mind Greed's company, as shown when she eats dinner near his seat at the table. Admittedly, this may have just been because she had simply gotten used to him during his time at the Rockbell residence.
 * Evidence against this would be the unease Darius felt as Wrath prepared his assault against the group in the room above Father's lair. However, this might just have been Wrath's manner as he prepared to attack, not necessarily his inhumanity.

2003 Series

 * The deaths of the Homunculi in the 2003 anime greatly differs from that of the manga versions. While the manga has them disintegrating into black dust, in the first anime, after death, their bodies seem to go into extreme rigor mortis before melting down into a puddle of Red Water (presumably the Red Stones that is their make-up).
 * The exact reason why the Homunculi are named after the Seven Deadly sins is never fully explained in the 2003 anime. However, they still embody the sins they are called after; while Greed and Gluttony have similar roles and personalities to their manga counterparts, there are examples of others living up to their namesakes:
 * Lust was born of Scar's brother's lover, and had affairs with other men, including Lujon.
 * Envy despises Edward and Alphonse for being the "favored" children of Hohenheim and for being abandoned because of them. He also hates Hohenheim for being able to find a life of peace and happiness even after abandoning him. Envy is not simply wanting other things, it is also the "hatred of others' good fortune".
 * With Pride, the aspect manifests through the power he wields from precognition, and having political power at his beck-and-call. Pride refers to himself as "God's guardian angel".
 * Wrath, is the desire for harm. He tried to kill Izumi, and tried to take Alphonse's body, as well as Edward's other arm and leg. Wrath can also be described as just "Hate" or "Anger". This aspect in the story may also refer to the wrath inflicted upon those who encounter the gate; the 'birthplace' of the child's homunculus form.
 * Sloth passes up numerous opportunities to kill the Elric Brothers and their friends. When she does harm someone, she often uses one arm, while the rest of her body is immobile. Her turmoil over her memories and feelings as Trisha could also be seen as embodiment of her sin, as the original definition of Sloth was "sadness to the point of despair". The motif of water that she follows may also refer to a current never moving against its designated flow or path.
 * It is never explained what would happen if there were to be more than seven Homunculi at a time. Since they're created from Human Transmutation, more than seven could theoretically be possible. Envy suggested that their 'master' could create more Homunculi at a time if required.
 * In the 2003 anime the alchemist Majhal claims to have failed a human transmutation, yet none of the Homunculi were credited as his creation. It's probably that he simply was not talented enough to open the Gate during his attempt, or that the Gate would not open because the person he wished to revive wasn't dead.
 * In the 2003 anime, Envy said that "It's been forever since the seven of us were united", implying that there were Homunculi before the current Sloth, Wrath, and Lust, minus Envy (who's the first one born), Greed, Gluttony, and Pride. Some of these incarnations, such as Envy, Greed, Gluttony and probably Pride are those present in the story's time-frame and have been alive for, at the least, a lengthy period since the original homunculus Envy was created. A similar comment is present in the manga and 2009 anime storyline, but there it's a clear reference to Greed's having rebelled before King Bradley ever became Wrath.
 * It's ironic the way the Homunculi ridicule humans for being so easily manipulated when Dante is doing the exact same to them with even less effort.