A Homunculus in Haunting Ground is an artificial human or humanoid created through alchemy. Lorenzo Belli created several incomplete homunculi, which now run amok in Belli Castle. Lorenzo's clone/son, Riccardo Belli, is a homunculus who created another homunculus, Debilitas, using a dog's placenta as a base, although Debilitas turned out to be a failed creation. A misconception is that Daniella is a homunculus when this has never been officially proven and official evidence suggests otherwise.
The incomplete homunculi can be found as deformed babies, or grey humanoids with white eyes. The "adults" are completely harmless and will do the same thing continuously (lick the floor, smash its head against the wall, admire art, etc.), unless disturbed by Fiona Belli. Then they will powerfully swat her away.