My actual preferred ABO worldbuilding headcanon (based on a lot of thought) is that almost all of them are intersex. They aren't gendered until they start approaching their first puberty. Up until that point they're usually called Pups or Children and use whatever pronouns.

Almost every one of them is born with some form of both "male" and "female" parts. The exceptions are Alpha Males who are only born with male genitalia and Omega Females who only have female genitalia. Everyone else falls somewhere in between.

When they hit first puberty is when their primary gender gets exposed. This translates to a change in the genital region. Males have more developed male genitals and develop more masculine bodies (thicker shoulders, more muscle mass, etc.) and the same with females.

However that's really where the changes stop. Betas don't actually change a whole lot down there. They're still very similar. There's not a lot of sexual dimorphism (physical differences in the body based on sex such as breasts on females).

The ones who go through the dramatic changes are Alphas and Omegas - together which only make about a total of 20% of the population with Alpha Females and Omega Males being even rarer due to low ability to reproduce.

Alpha Males are usually much bigger and muscular than Betas or Omegas. Their teeth are the biggest of the three genders and their jaws are powerful. They're aggressive when they're younger but generally mellow out unless those behaviors are encouraged. Their ruts are harsh.

Betas have the smallest teeth of the bunch and are much calmer of the three. They're the most common of the three genders but they can be just as bad as the others. They have both heats and ruts but they're generally mild and last about a day or two.

Omegas generally have it the harshest. They have wild personalities when their instincts are triggered and it's known not to mess with them. However, they are highly desired because of their ability to reproduce and their sex drives in bed. They're not really feminized though.

Their heats are extremely harsh and painful. It's better to have them with a partner of some sort rather than ride it out by themselves. They're usually quick to pair off and it's not uncommon for them to be mated and possibly with pups by high school.

Omega Males are usually seen as more sex tokens. All the fun of an Omega without the risk of pregnancy. There's a weird dichotomy of them being mocked for being Omegas and yet beloved for their bodies as well. It's not fun. They make up maybe 3% of the population.

Typically for Male Omegas and Female Alphas their secondary parts (penis and vagina respectively) don't really grow to be anything and shrink a little over the years as their other parts become more primary.

I have a whole bunch of other world building ideas that come from these ABO people and how they live differently but I'll end this thread here unless anyone's interested. I like writing interesting ABO worlds.

I will also accept questions because what better way to think of ideas than to brainstorm with people?