A homunculus (Latin for "little man") is a representation of a small human being. Popularized in sixteenth-century alchemy and nineteenth-century fiction, it has historically referred to the creation of a miniature, fully formed human. The concept has roots in preformationism, as well as earlier folklore and alchemical traditions.
References to the homunculus do not appear prior to sixteenth-century alchemical writings; however, alchemists may have been influenced by earlier folk traditions. The mandragora is one example.
The homunculus has also been compared to the golem of Jewish folklore. Although the specifics outlining the creation of the golem and homunculus are very different, the concepts both metaphorically relate man to the divine, in his construction of life in his own image.
While the Homunculus depiction differs from game to game, it basically retains the same traits between incarnations about it being an artificial humanoid created by external means, likely by the practice of alchemy or even by arcane arts.
In Castlevania Chronicles, they appear as tiny creatures that burst out of vials. In Castlevania: Dawn of Sorrow, they are human-sized, attached to breathing tubes, and can only be found submerged in water.
Homunculi are only encountered at the end of the first section of the laboratory, past the Frankenstein's Monster. They are initially found encased inside of little glass vials displayed on cabinets. As the player approaches, they will break from their tiny crystal prisons and start running around, shooting a small, swirling mass of energy at the player from a distance every once in a while.
Even though they die in only one hit, they can detect the player from far away and commence their attacks even before the player has an opportunity to get near them, not to mention their small size makes them difficult to target, so they should not be overestimated.
A humanoid creature with a single large red eye that dwells in the water-filled portions of the Wizardry Lab. It is anchored down by a tube that extends off its back and will remain immobile until the player approaches. When awoken, it will lunge out pulling the tube on its back tight. If not killed at this point, the tube will break, allowing it to chase the player all throughout the room.
After detaching from the tube, it will begin to change color. Once it turns purple, it will disintegrate.