Chimeras are created because of flaws that are endemic to the process of Binding and Devouring. In both cases, this involves a spirit taking control of the human's body, destroying the human's mind in the process. This usually drives the invading spirit completely insane, leading to the perceived danger and destructability of the chimera. A chimera's form can be anything, but it is usually incredibly monstrous, combining aspects of both the spirit's and human's forms.
In Binding, a spirit is forcefully bound to a human without the spirit's consent and held in place with special runic tattoos. However, this places the spirit's will and the human's will in constant conflict with one another. Each is trying to overpower the other and as long as the human succeeds this results in a Binder. However, if the spirit's will is stronger, the spirit overpower's the human's mind and moves to take control.
This process causes the spirit's mind to forcefully combine with the human's own, and this collision of minds is what usually drives a chimera insane. However, there are ways to ensure a chimera's sanity. If the spirit's and the human's minds are relatively the same before they merge, then the resulting chimera will be somewhat stable. Alternately, if a chimera is surrounded by the spirit's weakness for a long period of time, depriving it of its powers, it may begin to grow more stable. However the chance of success with either of these is incredibly unlikely, leading to few trying it.
Chimeras created through Devouring are somewhat different. In Devouring, the spirit's essence is consumed by the human, forcefully transferring its power and destroying the spirit in the process. However, this leaves a small portion of the spirit's mind inside the subject. Not enough to be considered sentient, just a small little voice in the Devourer's head. If a Devourer consumes enough spirits these fragments combine, forming a new spiritual entity that's almost always insane even before it overtakes the Devourer's mind, which just makes it even more unstable.
Homunculi are created when a spirit enters the Material Relam and begins Devouring humans for the energy they posess inside them. For normal humans this is minimal, but for pacters, binders, devourers and people with sparks, the energy within them is much greater, meaning they provide tempting targets for the ravenous spirit. As the spirit devours more and more humans, its inner nature begins to change. Slowly, it begins taking on human mannerisms. This may include purchasing food, taking on a more humanoid shape than it usually would and in some extreme cases, trying to court a human for a mate.
As the spirit continues to devour more and more humans, it tries to become more and more human. However, due to the large number of humans it has devoured at this point, it is unable to decide on what "kind" of human it wishes to be. This causes the spirit to begin assuming the personas of the people it has devoured, but since it cannot pick one it changes between all of them, never being able to stay with one form for long. Due to this process homunculi are usually as erratic as chimeras, if not as destructive. They may behave like an old man one minute and a bawling toddler the next. As far as anyone knows, this process of shifting never ends, leaving the spirit in a perpetual cycle.