I find eggs intrinsically hilarious
Ontos have five sexes: enig, seml, thed, obon and bren. Each sex can reproduce with any of the other four. This system was set up by their creator (or technically, the creator of their precursors) to maximise reproductive capability, as it meant that any given individual could potentially reproduce with 4/5ths of the population instead of just 1/2.
Only one individual in a reproductive pairing will lay the egg(s), depending on the combination of sexes. (For example, an enig will lay the egg when paired with an obon or bren. Seml and thed will lay the egg when paired with an enig.) Ontos generally take the surname of whichever of their parents laid the egg.
Some bonus information about ontos eggs is here.
Ontos have no sexual polymorphism discernible to a non-ontos. The only way you’d be able to tell is from the shape of the gonads, which are internal.
They don’t have genders as such, because of the lack of polymorphism. There are some stereotypes about how each sex behaves, but this is viewed more along the lines of our horoscopes (everybody thinks it’s rubbish but some people have fun with it anyway). Ontos accept that they will probably get labelled as either male or female by non-ontos, and most of them just go along with it and don’t really care unless they find a way to use it to their advantage or to be funny.
Agriltheans are mollusc-like. Individuals produce two types of gamete and during reproductive sex can swap both kinds so that both participants can lay eggs. Agriltheans cannot self-fertilise. They do sometimes argue over which participant has to lay the eggs. Many couples lay some eggs each, in the interests of fairness. Others hold the opinion that the one whose idea it was that they reproduce should be the one to take on the burden of egg-laying.
In the past, agriltheans were mostly r-selectors. They laid large clutches of eggs, and left them to manage by themselves. After a couple of years, any young still alive would be taken in, looked after and raised. As the agriltheans evolved sentience and developed civilisation, they decided it was cruel to bring so many young into existence that were doomed to die, so they switched to being more k-selectors. They still lay large clutches of eggs, but as they lack the resources to raise them all, they set aside the ones they judge to be most hale to be raised, and eat the rest before they have a chance to develop. They lay slightly fewer and bigger eggs than they did at the beginning of their recorded history, demonstrating that they are evolving further down the k-selection path.
Agriltheans do not quite understand the concept of gender as it applies to dyadic species. They can understand why it might be necessary to have some way of telling others ‘I make gamete p and need someone who makes gamete q to reproduce with’ (although that sounds frightfully inconvenient to them), but not everything else it entails. Most of them much prefer to be referred to by non-gendered pronouns so as to opt out of the system.
Melmarus chooses to identify as female because her dear friend Serick is one, and Melmarus wanted to be the same as her friend.
Rathameldhontel are dyadic-ish. Their sex is not genetically determined but environmentally. Like crocodiles, they will be male or female (to use the human equivalent terms for the sake of brevity) depending on at what temperature their eggs were incubated.
Eggs can be laid (where they will be relatively cool), or kept inside the body (where they will stay much warmer) until just before hatching. Many individuals will choose to just lay all their eggs and then use magical means to curate the temperature.
Rathameldhontel sex differences are not clear-cut - eggs that were incubated at the boundary of a temperature range tend to become individuals with middling characteristics. All rathameldhontel have a somewhat uterus-like organ that may be fully functional (able to gestate eggs), so small as to be non-functional for egg-holding, or somewhere in between (able to gestate maybe one or two eggs at a time). They also all possess a protractile genital that may function more like a wilkins (transferring gametes into the body of another individual) or an ovipositor (laying fertilised eggs for external incubation). In general, ovipositor + large egg chamber tend to be found together, as do wingus + small egg chamber, but this is by no means absolute. Any of these variants can occur in combination, and many rathameldhontel tend to see themselves as mostly one sex rather than only one.