The chaos-ridden are beings with a piece of the void—chaos magic—in them. They were created by experiments performed by the Enemy of Death, who placed chaos into living creatures, and has since used them as servants and for his army.
While animals can have chaos put into them, human Chaos-ridden are made from the freshly dead. Animal chaos-ridden react to chaos differently, as they do not die when chaos is placed in them; on the other hand, there is still no way to safely place chaos into living humans.
In the case of chaos-ridden creatures made by a chaos mage, the chaos robbed them of their speech (though they are able to communicate with their Master telepathically) and intelligence; though not entirely mindless, they have lost their own free will and ability to feel, though they do respond to their Master's needs. They blindly followed and worshiped the Enemy (or presumably whichever chaos mage created them), and can also be controlled by the Enemy's servants, such as Joseph, if he commanded it. The transformation also makes them bloodthirsty, more aggressive, but also stronger and faster, capable of inhuman abilities. Technically undead, the Chaos-ridden are hard to kill and no longer have a natural manner of death, i.e. they no longer age and are practically immortal, though not exactly living.
Chaos-ridden animals are still capable of breeding. Animals that were born chaos-ridden, like Havoc, never knew the commands of the Enemy and still have feelings and personality—or characteristics that distinguish them from other chaos-ridden creatures—though they still can be commanded by or respond to a Makar—Callum Hunt, in Havoc's case. Animals like this are apparently smarter than other animals.
It is shown in The Silver Mask that the Alkahest can safely remove the chaos from a chaos-ridden animal, or at least the offspring of chaos-ridden. They become a normal animal completely.