How does interbreeding work in your campaign?

[Viking Bastard]

If it's fey, it can breed with anything. This also goes for goblins, who are long-since exiled fey. Goblin half-breeds generally come out more or less goblin. All goblinoids are of the same species, some are just born different from the others (bigger and stronger).

This may or may not be related to goblin females being hardwired to be attracted to strong alpha males and that a lot of them live in or around troll-infested mountains.

 

[thastygliax]

Going by the examples in the core rules, I usually assume the following in my games:

* Humans can interbreed with elves and orcs, producing the standard PH hybrids. The reason why they can are a bit murky, and may involve common ancestors and/or magic in the creation of one or more of those races. The only other standard races that can interbreed are half-elves with elves and humans.
* BTW, I still use AD&D 2E's simple rule for determining the race of a human-elf hybrid: If 50% or more of the character's ancestors were elves, they were a half-elf; if fewer than 50%, human.
* Dragons capable of polymorphing can produce half-dragons, per MM.
* Many outsiders can interbreed with Material Plane races, resulting in half-celestial and half-fiend hybrids. At least among human-outsider hybrids, once the outsider blood is diluted further, you end up with planetouched.
* A number of MM monsters (such as owlbears) must be the result of magical experimentation. In one of my campaigns, I created a wizard villain who did such work, and created a new spell, accommodation, to enable it.

Depending on the game, I then borrow some or all of the following from other sources:

* Some elementals, and outsiders with an elemental subtype, can interbreed with Material Plane creatures, creating half-elementals (Manual of the Planes). Once that extraplanar bloodline is diluted, you get genasi (Monsters of Faerun).
* Green Ronin's Advanced Player's Manual has a template for adapting any material creature to become an aligned or elemental planetouched, which allows for much more diversity than standard planetouched or genasi. (That book also has rules for building new races, including new hybrids of PH races, but I've yet to try them out.)
* GR has also published Aasimar & Tiefling (new halfbreed races, some of them extremely exotic) and the Advanced Bestiary (new templates of all kinds). I only own the latter, which contains a few hybrids (half-drow and half-giant, for example) plus the ultimate hybrid generator, the Amalgam (combine any two monsters to create a new one). The latter is easily the most complicated template in the book, but if you want to go there, it's exactly what you need.
*Stormwrack includes some new subraces, such as the aquatic half-elf.

In my games, I rarely use all the demihuman subraces from the MM--deep dwarves are simply PH dwarves, I only use the standard PH halfling, and I often ignore gray elves and forest gnomes.

I normally limit my players to races with no Level Adjustment, or +1 at most. This rules out half-celestials, -fiends, and -elementals, though NPCs of those hybrids exist. (The whole concept of LA tends to give most of my players headaches anyway.)

In the campaign I'll be starting soon, I'm being more generous than usual about the range of races I'm allowing. For example, I'm allowing aquatic half-elves and the APM's planetouched template (LA +0 for elemental, LA +1 for aligned; one PC will be a water PT human). But I'm still barring races that are hard to explain even in the melting pot of GR's Freeport setting.

 

[malcolypse]

A few options that I've come across or used in my games in the past:

As a player, I once had a halfling (who due to world-shattering shenanigans over the course of the campaign had become half-celestial) character fall in love with, marry, and have kids with an elven sorceress. The DM, in the post game discussion described how with his unnaturally extend lifespan, my character became the source of a new race, which other players gleefully named three-quarterlings.

Also as a player, I'm currently playing a bugbear in a game where the world is literally less than 100 years old, and the races are newly created, and only two generations have born since the gods let the first generation loose on the land. Another player has made suggestive comments in character about my character and the halfling that my wife was playing. The response: "Why would I honor a tiny, pale-skinned, practically hairless, and most certainly too fragile creature in such a way? It's disgusting and insulting. Nothing against you personally, of course, but you and your species are a monstrous indiscretion blighted upon the land by an obviously mad and cruel deity."

As a DM, I'm generally against it without magical assistance or an incredibly inspirational character background. It is always the exception rather than the rule, because if there's enough difference between two species to warrant different stats, then they're probably too far removed genetically to qualify as races of the same species IMO, tions and ligers and bears (...which is what you get if you crossbreed a bear and bat. Terrifying on many levels, but unfortunately since both begin with the letter b, it is difficult to differentiate them from their parent species in writing.), be damned!

 

[Gold Roger]

As a rule there's no crossbreeding without powerfull arcane magic or divine intervention (no psionics and primal magic cannot create anything unnatural).

Exceptions are half-orcs, dragonborn and yuan-ti. Orcs and humans once where one race. Dragonborn where once a human tribe that "stole the power of dragons" and produce dragonborn offspring with humans. Yuan-ti are the descendants of an elder snakelike race that escaped extinction by crossbreeding with humans using unholy magic, which is also why they are so unstable and prone to mutation.

Players can still play half-elves, tieflings or genasi, but these are legendary, propably even unique, beigns or something other than crossbreeds. In fact there are some impostors and con-artists who claim to be one of the fabled half-elves (Usually humans with false ears or elves with false beards).