Numerous characters of mixed parentage appear in DMFA and its fan-created derivative works.
Amber is addressing the issue of crossbreeding in a Demonology 101 bonus mini-arc entitled "Hybrid Genetics".
Many of the races in Furrae can interbreed. However, a multiracial pairing will not necessarily produce offspring, even if the two races are technically compatible. Race, species, and luck all play a role. Furthermore, the magical natures of most Creatures can throw a wrench into the equation.
According to Amber:
Magic makes things much more complicated, and tends to fill the gaps where simple genetics would fail. The general rule of it can stand that if the odds are there for something strange to happen, it is very likely it has happened.
Since this seems to confuse many people, it is worth mentioning that the Hybrid Genetics series explicitly states that a pairing between a Creature and a Being is not guaranteed to produce another Creature as their offspring. This is particularly evident with Angels, who all too often seem to have a Being child.
As a general rule, the offspring of a Creature-Being pairing take after their Creature parent in terms of racial abilities and their Being parent in terms of appearance. For instance, Daniel Ti'Fiona is an Incubus who inherited many of his physical characteristics from his father.
Furrae's legal system does not recognize hybrid Creature-Beings as distinct groups. Those with strong Creature traits are generally as classed with their parent Creature(s). Not all of them qualify as such, however. Those with few Creature traits typically fall under the Being umbrella, although they may possess some magical abilities or physical traits which set them apart from other Beings.
It's not unknown for children of Being parents to be born with Creature traits such as extra eyes or limbs, either as the result of Creature ancestry or random magical influences. Some Adventurers encounter magical energies that can affect future progeny.
In practice, Beings who exhibit traits traditionally associated with Creatures may experience discrimination regardless of parentage. Only recently has Furrae seen a significant increase in tolerance and cooperation between Beings and Creatures.
Angels and Demons
Angels, Demons, and Cubi can easily interbreed amongst themselves and with most Beings. People jokingly suggest that the genesis of the Cubi race involved Angels and Demons getting along "too" well.
Unlike Cubi, children of Angels or Demons may inherit a partial repertoire of Creature traits. For example, Lorenda Soulstealer (the child of Kria the Demon-mare and a Being cow) possesses her mother's demonic strength and appetite, sharp teeth, and the ability to harden her hooves into sharp claws, but she lacks full-sized wings (Although this may have to do with a side-effect of Kria's being charm). She nonetheless counts as a full Demon for legal purposes.
While Angels can cross-breed, their offspring seems to be Beings for the most part, rather than Angels or half-Angels. Apparently this was not always the case.
Cubi can successfully crossbreed with Angels, Demons, Dragons, certain types of compatible Mythos, Were, and selective kinds of Gryphons and Phoenixes. What is produced in the crossbreeding largely depends on the strength of both participants, much like Cubi clan genetics. Predominantly, the race of the child is determined by which parent is the strongest, which is why Beings are primarily targeted for unscrupulous purposes. The offspring of a Creature more powerful than the Cubi participant will result in a non-Cubi child.
Half-Cubi phenotypes do not exist in canon DMFA. A child with only one Cubi parent will either be a full Cubi or inherit no Cubi abilities at all. (The latter may, however, display slightly greater magical potential than other members of their race.) Gareeku Manoko, a fan-created character, represents a non-canon exception to this rule. On the other hand, it is possible for full Cubi of mixed ancestry to have inherited traits from their non-Cubi progenitors. The origins of tauric Cubi lie in Mythos-Cubi pairings.
The Were race is less flexible in its crossbreeding compatibility, partially due to the fact the Were race prefers to keep its distance from other races. Only Cubi, Dragon, Mythos, Phoenix B, and Beings can procreate with Were individuals.
As the Mythos race comprises such a wide spectrum of sub-races, one cannot generalize about their ability to crossbreed with other races. Some Mythos can. Some Mythos can't.
Gryphons and Phoenixes
Gryphon A and Phoenix B can interbreed and produce hybrid children, although such pairings are uncommon. Since Phoenix B are essentially similar to Beings, they are capable of breeding with a wide variety of races, such as Dragons, Mythos, Cubi, Beings, Were, Angels, and Demons. However, the Phoenix B parent's ability to transform into an alternate form is skips the hybrid child's generation.
One Dragon-Being pairing appeared in DMFA. However, no further details about their relationship or the nature of their offspring have surfaced. Amber has indicated that crossbreeding with Dragons involves "tricky complications".
Dragons are surprisingly flexible with their breeding compatibilities. Despite the apparent feud between Dragons and Cubi, the two can reproduce, as well as Beings, Weres, Demons, Angels, Phoenix B, and Mythos. All Dragons in Furrae have the ability to switch between two forms: their base Dragon form, and a smaller form reminiscent of a Being, which makes interaction with other races less convoluted.
According to page eight of the Hybrid Genetics Mini-Arc, the Dragon gene is dominant and will override the possibility of the hybrid child being a Dragon. Hence, most children produced from relationships between a Dragon and a member of another race are Dragons, but the strength of the child depends on the bloodline - as more and more generations are formed, the bloodline becomes diluted, and any future offspring become substantially weaker than that of a pureblooded Dragon. Combined with the fact most Dragons are highly fixated on maintaining racial and bloodline purity, the creation of Dragon hybrids is often discouraged, and does not occur often, with a few exceptions.
The magical and enigmatic Fae do not reproduce sexually--at least not in the traditional sense. Fae are immortal, but they can die voluntarily. The soul energy is then recycled into a newborn Fae.  Some Fae have traditional family units; Miss Mab had parents who appeared in a brief flashback, and Queen Nutmeg and Albanion requested one of their souls for a child to raise as their own.
Mer and Insectis
Neither the Mer nor the Insectis can breed outside of their own races, with one exception. Insectis and Mer can possibly crossbreed, as they are believed to be genetic kin who adapted to different environments.
Creatures Unable to Crossbreed
The Undead become sterile after death and can no longer reproduce sexually. They create more of their own kind by inflicting a lethal bite wound on a Being or susceptible Creature. If dead within 24 hours of such an attack, the victim will rise as a new Undead.
Species (the base animal of an anthropomorphic character) plays an important role in determining how easily prospective parents can interbreed. Compatibility increases as a function of the similarity between species. Likewise, even Beings or Creatures of the same race may have trouble conceiving a child if their species are sufficiently different. Thus, two felines could easily reproduce, two mammals would have a good chance, but a mammal and a reptile might have difficulty.
With respect to species, children tend to take after either one parent or the other. Mixed Creature-Being offspring typically follow the rule that children take after their Being parent in physical appearance. For example, Lorenda Soulstealer is a cow like her Being father rather than a horse like her mother.
At present, DMFA has only one canon example of two Beings of different species having children. Seth Gianna (a kangaroo-rat) and Moira Gianna (a kangaroo) currently have seven children. Jyrras Gianna, their only male child, appears to be a kangaroo-rat like his father. All six of his sisters appear to be full kangaroos like their mother. This case of children's species following their same-sex parent may generalize to the rest of the population (and possibly to multispecies Creature families as well).
Notable Characters of Mixed Parentage
- Jyrras Gianna (Kangaroo Being mother/Kangaroo-rat Being father)
- Abel Rewanz (Feline Being mother/Feline Cubi father)
- Lorenda Soulstealer (Demon-Mare mother/Bovine Being father)
- Daniel Ti'Fiona (Feline Cubi mother/Feline Being father)
- Amber's post on species
- Hybrid Genetics - Angels
- Amber's post on Creature-Being trait inheritance
- Cid's explanation about Adventurers' children in Abel's Story (Page 4)
- Demonology 101 entry for Cubi
- Hybrid Genetics - Angels
- All mentioned in the DMFA Hybrid Genetics Mini-Arc, seen here.
- Amber's post on Beings of Cubi ancestry
- Information found in the Hybrid Genetics Mini-Arc.
- Amber's post on crossbreeding
- "Albanion" on Fae reproduction.
- See page six of the Hybrid Genetics Mini-Arc.
- See the fifth page of the Hybrid Genetics Mini-Arc.
- See the seventh page of the Hybrid Genetics Mini-Arc.