Do familiars reproduce? If 'yes', what kinds of offspring result?

Question

The spell Find Familiar summons a simple yet common creature of the caster's choice (like a cat, rat, raven or weasel). Such a creature's organs all function healthfully, including eating and sleeping. This creature is not a beast however:

Appearing in an unoccupied space within range, the familiar has the statistics of the chosen form - though it is a celestial, fey, or fiend (your choice) instead of a beast.

Can a coupling of (matching) summoned familiars reproduce? If 'no', why so? If 'yes', what offspring is produced (a 'common' beast, some 1/2 beast + 1/2 spirit hybrid or one of the three types of spirits)?

Answer 1

The rules don't specify, so it's up to the DM to make a ruling

If you are the player, raise the question with the DM and see if they want to explore this idea to its logical, or illogical, conclusion.

If you are the DM, you decide: does this intrigue you, or is this a hole down which you'd rather not go? If you do you opt for this, you need to carefully assess the "power" of these part-fey/part-mundane offspring. You are in home brew territory.

• Example 1: Fey-Kittens
  A cat familiar mates with a mundane cat, which later produces a litter of unusual kittens. They are more sensitive to fey creatures, and (for a 5e example) might have Advantage to Perception checks rolled to detect or sense fey creatures. (I saw something similar to this in an AD&D 2e campaign; kittens that were sensitive to unseen things, sold for a premium price, and they liked to chase brownies).

• Example 2. Fey-Ravens
  A raven familiar mates with a mortal raven. All offspring from the next batch of eggs are a bit more aware. In a 5e setting, let's say that they gain Common language proficiency similar to Kenku's. (See Volo's Guide to Monsters for the limitations on that). Or, the ravens (which can per the MM mimic some sounds already) gain Common language proficiency but their INT (2) limits their speaking ability ... though they respond with a cryptic "Nevermore!" to any rhetorical question. (Yes, we had a DM provide us with a talking Raven in a AD&D 1e campaign who did that for comic relief ... at least, it amused the DM to no end).

What the DM needs to consider is: if you offer this, how will your players exploit it? One thing you can be sure of is that the players, if offered a new toy, will find unique and often surprising ways to play with it. It could be fun, or it could become a nuisance. Given how few HP a familiar or small/tiny beast tends to have, they are easily killed off.

And, the DM can always rule that famliars 'don't / can't / don't-want-to breed' and be done with it. Given the telepathic link between wizards and familiars, at some tables this can be the kind of line that some players don't want cross - does the wizard really want to be telepathically connected to a mating cat? (See Lines and Veils)

Your follow up comment ...

All the other organs in a familiar are healthy and work normally. - hence the need for food, sleep, air and so forth (see link, above). Why would reproductive organs NOT function?

... can be addressed simply: the physical form, to include reproductive function, is all beast and thus there is nothing unusual about their offspring: any offspring are mundane cats, ravens, ferrets, etc.

Discuss with your DM, or, as DM, make a ruling.

Answer 2

Familiars are one of three creature types: celestials, fey, or fiends.

As this is their type, one could propose they can reproduce in the same way others of this type do. There isn't much documentation on the mating habits of these creature types, but there is some precedent in the fiends' case, since Cambions are defined on p. 36 of the Monster Manual as

...the offspring of a fiend (usually a succubus or incubus) and a humanoid (usually a human).

I suppose the existence of the Fey Ancestry trait in Elves and other humanoids could be considered evidence of descendance from the Fey, conceivably through mixed breeding.

As for the Celestials, unlike the elves, aasimar are, according to p.104 of Volo's Guide to Monsters:

...descended from humans with a touch of the power of Mount Celestia.

No mention of ancestry, or any other indication of cross-breeding.

Regarding the question the OP references, the accepted answer deduces that familiars need to eat because not needing to eat is a known trait and is explicitly specified for creatures such as undead, hence if familiars did not need to eat, this would be specified.

This does not imply organ function, nor even necessarily the existence of organs. One can argue that they require sustenance, but there is nothing to indicate that they metabolize that sustenance any differently than any other creature of their type, let alone in the way their chosen beast form would.

All that is stated is that they have the statistics of the chosen form, which is defined as synonymous with the creature's stat block. There is no reproductive data in the stat blocks of the beasts listed in the Find Familiar spell.

Answer 3

Rules for something as specific as this are unlikely to appear in D&D 5th edition, so as the previous answer correctly notes, this is up to the DM.

There is a little lore on this topic in the earlier D&D 3rd edition, although it only partially answers your question.

Celestial animals appear to be able to reproduce. According to Races of Faerun p.112:

Aasimar are the descendants of humans and some good outsider, such as a true celestial, a celestial creature, couatl, lillend, or even a servant or avatar of a good deity.

"Celestial creature" here refers to a template applied to a creature, often to create the stats for a celestial animal, but can also be applied to a humanoid (e.g. according to Complete Divine p.110, Corellon has celestial elves in his service). As per Manual of the Planes, celestial animals are often created from the souls of dead humanoids arriving in the Beastlands, and perhaps also from the souls of animals.

Races of Faerun p.112 describes the result of such interbreeding:

The child of an outsider and another creature is a half-fiend, half-celestial, or half-elemental.

By this definition, a celestial dog plus a regular dog produces a half-celestial dog, which would be more powerful than either of its parents. However, as per the 3.5 Monster Manual, half-fiend and half-celestial templates are applied only to creatures with an Intelligence score of 4 or higher, which rules out normal animals.

Half-fey, Fiend Folio p.89, have no such limit. They are strange versions of normal D&D creatures with butterfly wings and resistance to enchantment.

However, I suspect that the intent of the half-celestial, half-fiend, and half-fey templates in D&D 3e are for the likes of more powerful outsiders interbreeding with humanoids, rather than celestial animals with other animals, since the latter creates the paradoxical situation where the offspring is considerably more powerful than its original form.

When you consider the actual power level of a celestial animal as a 5e familiar, it has no innate power above a normal creature of its type, aside from its connection as a familiar. It seems unlikely that the cross-breeding of such a celestial ferret with a regular ferret would produce powerful offspring. What I'd expect is more like what Races of Faerun p.112 notes happens when e.g. an aasimar breeds with a human, which is a 50-50 chance of being either an aasimar or a normal human, with the possibility of the celestial bloodline remaining dormant and re-surfacing generations later.

In short: D&D 5e has no rules on familar reproduction, and while D&D 3e lore suggests that celestial, fiendish and fey creatures can indeed interbreed with regular creatures, the exact nature of their offspring is unclear, particularly as D&D 5e's celestial/fiendish animal familiars are weaker than their 5e counterparts.