Some hellhounds are born with 'mutations', but these are not random mutations in the genetic sense. Someone, somewhere, planned and designed each of them and put the potential, with a very low probability of being expressed, into the hellhound genetic template.
Skraahiye mutation as seen in Chokedamp gives the hellhound a scaly texture.
An extra heads mutation is seen on Cerberus.
mindbonds in the same way as demons, except that a hellhound bond has an imbalance of power in favour of the demon. Hellhounds are designed to be obliged to bind at an early age.
See Mindbond (Demon) for more information.
Hellhounds breed like dogs instead of simply 'appearing' as demons do. It's likely that this is a deliberate feature intended to control the rate of reproduction.
Because of the demonic genetic material, incestuous breeding does not carry the risk of adverse effect to the offspring. Nevertheless, most hellhound bitches will not breed with a close relative.
Hellhounds are still compatible with the dogs from which they were adapted, but halfbreeds are not 'one quarter demon'; the demonic genetic material is 'dominant' and tends to overwrite most of the DNA from the dog parent. The result will be almost fully hellhound, but with some dog characteristics (considered weaknesses).
There is speculation that magic usage may be affected, since dogs are believed to have no magic ability. Halfbreeds may even be able to bind to non-demonic creatures.