Featured in both FullMetal Alchemist manga and FullMetal alchemist anime, even featured in the FMA film Conquorer of Shambala. In both manga,and anime Homunculi are near immortal and are the enemies of Ed and Al.
ANIME DESCRIPTION: (NOTE ANIME DESCRIPTION OF HOMUNCULUS IS VERY DIFFERENT) In the anime FullMetal Alchemist the Homunculus are the result of alchemy's taboo human transmutation. Their birth is both sad to them themselves and to their maker/creator. They are born when a human/alchemist attempts to break the flow of life and death by using human transmutation to bring a deceased person back to life. Alchemy isn't perfect and neither are the alchemist who use its power to remake things. For this reason the transmutation fails violently maybe its because humans are to complex to create/ recreate through alchemy, maybe its because humans/alchemist can only recreate lesser beings(chimeras) and still the transmutation is flawed at best, or maybe its simply god/nature itself that shows it fury on the alchemist. As a result transmutation pulls in the alchemist performing the transmutation often if not always taking body parts from the alchemist such as (eyes, a leg, reproductive organs, lower abdomen organs, etc). And even then with the loss of the organs to the GATE the alchemist must come to face the shockingly horrifying truth, as they gaze at a massive clump of meat,organs misshapen in what looks like a horrifically mutilated version of the person the alchemist was trying to bring back, now a homunculus. The homunculus is born with a mind and a distorted body at best but with no soul. Due to this the homunculus is extraordinarily weak and will die shortly after being born. The way for a homunculus to survive being born is if he or she feeds on the essence of life itself, via red stones which are incomplete philosophers stones.
By feeding on these red stones the homunculuses lifespan is expanded by feeding on enough of these stones the homunclus begins to heal itself from the incomplete transmutation that gives them life, and thus acquiring the form of the the human that the transmutation failed to bring back. As a side affect the red stones activate dormant memories of the deceased person again in part due to the fact that while the alchemist failed to resurrect the soul however they did manage to create both an imperfect mind, imperfect body which by eating human lives the homunculus's body is repaired. As a side affect of eating the red stones the homunculus body acquires "red veins" that are exterior. It is believed that these veins act as a circulatory system for the red stones protein. Also noted by stabbing all the end points to this "circulatory system" the homunculus is greatly weakened and can be completely immobilized if this is systems flow has been stopped and the homunculuses seal of oroboros has been damaged. Again a side affect of eating the red stones. Homunculus can cheat death by using the lives within the stones although they appear immortal due this there is a catch should the stones be expelled from their stomach such as Lust suggested to wrath they'll die permanently. Despite having fragmented memories and a mind the homunculus are by no means the human the alchemist tried to bring back. They hold a great hatred towards the alchemist that made them "Thank you for this incomplete body and this incomplete life and for not giving me a soul" (wrath), many homunculus try to prove that they have their own identity like Sloth did by trying to kill Ed, Al proving she wasn't their mother as no mother could kill her children.All homunculus a fake not the real thing as such they are greatly weakened when near the remains of the human the transmutation failed to bring back. Dante went as far as creating a fail safe for renegade homunculus one involved using the skull of Greed's counter opposite plus a complex array of transmutation circles to seal Greed away in a blissful slumber. The other involves way of sealing a Homunculus using the infamous FMA cross with a serpent and wings
drawn into a complex array this combined with the remains of the person the transmutation failed to ring back not only weakens the homunculus but also forces it to vomit fourth the stones within the stomach as Greed would say "My life!! That's my life!!" after being expelled the stones returned to their red water state from which they were originally created from. Another weakness the homunculus possess is their elemental malleability this is due to three different reasons. First the red stones act as alchemic amplifiers meaning the stones themselves have some alchemic leeway two having been born artificially from alchemy their body is more "expandable" as there's no other way of putting it, and three all Homunculi have seen the GATE the source of life/alchemy. Ironically homunculus have a greater understanding of alchemy yet are incapable of performing it. With two exceptions, the first being Wrath as he possess Eds right arm, and left leg he took it during the transmutation when Ed gave his leg to bring his mom back to life, and when he offered it to the gate to get Al's soul back. The second homunculus to be able to perform alchemy is Greed he however can only do alchemy on himself by altering the carbon bonds in his density. Also because of their malleability a skilled alchemist can rework their bodies without a human transmutation rebound. In the movie "FullMetal Alchemist:Conqueror of Shambala" wrath states that homunculus' true purpose is a catalyst for opening the GATE. Homunculus also have one other weakness the seal of oroboros should the seal be taken via alchemy as lady Dante did to Gluttony the homunculus will loose its mind and become a creature of pure instinct. Because of this Gluttony could not comprehend his surrounding and in the end ate Dante.
Later in FullMetal Alchemist Conquoror of Shambala it is discovered that Gluttony some how mutated becoming the living incarnation of Gluttony itself. It is unknown and unstated as to why this happened. But what is known is that gluttonys blood was pseudo plhilosophers stones and was so full of this substance that his skin started to bleed it and when his "blood" touched any surface red stones were created. Also Gluttony had a hunger for Homunculus as he went after Wrath but ignored Alphonse.