Every culture has its myths and myths always fit in with the culture they belong to. From the Native Americans and their stories of respecting nature to the blood and brutal of the Aztecs. If I had the myths of Chinese mythology in the Norse world it would feel off.
Needless to say the myths a culture has are important to keep the feel of the world. How can I make sure that the beliefs and myths of the culture fit in with the culture itself?
Myths and other similar details, give a setting depth. They are in essence the nitty gritty of culture, the grain of sand level detail that really makes a world come alive.
That necessitates a few pre-requisites before you can come up with myths that make sense and feel organic. Ask yourself the following and myths, history, religion and culture will start creating themselves so to speak.
Questions:
What is the planet like. Is it earth-like? How many planets are in the solar system? Is plant life green, the sun yellow and the sky blue? What flora and fauna are important to the people? Weather matters as well, many of Earth's early deities were tied into aspects of weather.
Cosmology, the actual cosmology is less important than what people believe to be true. How was the world created and why. Who created the world and people? Is there one god or many, perhaps none? How does the divine interact in the world? What does the deity require of worshipers?
Sentient beings. Are we talking humans or something else? How we see the world depends on who we are. If you have a race that is blind, for example, color won't play into myth (as it so often does).
Culture/Society/Civilization. You seem to have this pretty well locked down at this point with your migratory blimp people. What natural phenomenon will be important to them based on their lives? What are the challenges they face? What is the worst situation a group or individual could end up in, i.e. what are their fears and conversely hopes. What makes a 'good death' and 'good life'.
Resources. What resources are important to them, how do they get said resources? What are the challenges and why is the resource (or resources) important to them. If they use gas in their blimps for example the gas could be the "breath of [insert deity name here]"
Heroes and villains. What historical figures have impacted their society and how did they do it? What is the acutal story versus the myth?
Tribal differences, how do people diverge (if you have more than one group). How do their myths differ and why?
Once you have all these things locked down you can develop a myth for the culture. In many ways you can make a myth what you want it to be but you should keep the setting in mind and ensure that it doesn't contradict how a person in the situation defined would think.
Examples: Here is an example. (your world...)
The people are constantly on the move. Things that impair or facilitate that movement are of primary concern. Several myths can develop from this. The obvious myth is a heroic myth. The hero that came up with their floating civilization, or the hero that saved a floating city from destruction. Pick your poison here, it could be a villain myth attached or maybe a natural disaster.
Cosmology would likely revolve around a sun deity, the obvious path here is that you must constantly follow him to stay alive, which is quite literally true in this case. The religion just kinda creates itself now doesn't it?! The antithesis of course would be darkness and some deification of the chasing darkness, the idea is ripe for a cult...sacrifice to @bowlturner...err the dark lord.
Notes:
How myths show themselves in your world is important. There can be storytellers and priests that share lore and law. Sayings that don't make sense to outsiders or that most don't know the origin for.
Resources:
My personal favorite resources is The Hero with a Thousand Faces. Joseph Campbell is a great read for world builders and I highly recommend it even if, academically speaking, some of his theories are arguable.
A good myth is going to focus around a common question that your culture will ask, or will have difficulty with. It will provide guidance and explanation in the form of symbolism and important characters. In your culture, it will deeply resonate with your individuals and, the myths that survive best will be the best stories, the ones the kids want to hear again and again.
Find a question that your society is always going to ask. "Where did we come from?" "Why are we different from those other people?" "What is special about [common cultural thing]?"
Use your culture's famous symbols, people, or events in a story, which explains the answer to the question and shows what is important to your people.
Make sure the story correlates to your culture - If the evil bad guy was a generic bull, bulls are likely unpopular animals to this culture. It wouldn't make sense to worship them.
I think this is a chicken and the egg type thing. Myths, go hand in hand with the culture that spawns them, and in turn help mold that same culture.
So the more you know about the culture you want to create myths for the easier the myths will be to create, and the better your myths for a culture are the better you can refine the culture from the stories.
So if you know how a typical person from a culture should react to a given situation (being culturally driven) then there are likely stories in that culture praising/rewarding that same behavior. That is a good place to start. Or on the flip side, there might be punishments/ostracizing should someone NOT perform a certain way. Some cultures will have more negative lessons others will have more positive lessons.
Also the more you know about the history of the culture, taking some of the known 'heroes' and 'villains' and stretching their exploits and making them larger than life is another great place to go. Most myths have some kernel of truth in them, even if it's just an ideal to be passed on.
Oh and villains, tend to represent what the culture considers to be bad traits the heroes ideals to aspire to. So a hero in one culture can be a villain in the next.
My advice for creating a system of myths would look something like this:
What religions, beliefs, etc. have you already laid out? This might determine the theme of the myths. For an abstract example, in a religion with good and evil, moralistic fables might be common, or mythical creatures might be fearful demons or, conversely, benefactors.
What is familiar to people, and assumed to be a given? The people might assume the characters in their myths are like them, or they may be set apart by the fact they don't do something.
What is scary/mysterious/unknown/unlucky in this culture? Some myths might stem from that. Also, many, many myths serve to explain what science can't. If the people don't know about something, they'll assume whatever fits for them.
Think of cultural traditions and manners. Might myths factor in to this? In Ancient Greece, the gods could disguise themselves as humans, so people were hospitable to strangers because you could never tell if they were a deity in disguise.
What creatures are there in your world? Are they a danger? Do they provide people with something they need? What do they mean to the people? Predators are often seen as fierce, and can be either wicked or noble. Animals the people need to survive often represent fertility. How your world views animals can affect legends and myths.
What are the morals of these people? Ultimately, this decides how the stories are told. What qualities are praised? What is taboo? How rigid is the society's power structure? The last one is important in that a rigid society might have stories with the connotation of "know your place," while a society with more opportunities might have a few "rags to riches" myths.
What aesthetic do you want your myths to have? Myths can be biased. They can praise, instruct, or simply entertain. Myths can be full of imagery and detail, or stripped down to the bare bones of its intention. What time period was this myth made? How did it come to be? How did it evolve over time? Was the meaning changed to suit society? Where parts taken out because of taboo, or something else? Another part of this is figuring out how symbolic you want the myths to be. They can be legends of everyday heroes, or they can be seeped in the culture, assumptions, and associations of the people so much that a foreigner would scratch their head at the meaning.
Hope this helps!
Creating believable myths goes hand in hand with creating a believable culture and a believable history.
The first thing to consider is the functions myths fulfil:
A good myth will fulfil as many of these functions at the same time as possible.
So where do those myths come from?
The first thing to look at is the physical conditions those people live under. From their point of view, there is no such thing as a year, there's only one period, the solar day; however perception of the day will be more like the year for us.
Since those people migrate with the light, they will not know the night, nor the stars. For them, the sky is where the sun shines, and darkness means death. They will fear the darkness, since they know that out of darkness, only death comes. They won't see the living creatures adapted to darkness, because those never see the light; however they will see deceased creatures of darkness, and they will see creatures of light that went into darkness and died. They will also know that where the world comes out of darkness, it is cold; indeed, it's probably dangerous for them to even come close to that region, due to the coldness there. Since from the dead bodies they know there are creatures on the dark side which undoubtedly look gross to them, they probably will see the dark side as a world of evil and death, and the bright side as the side of the good and of light.
The main question of their life would then be, why is there a dark and a bright side, and why are they moving around the planet (since they migrate with the sun, they will be aware that going round the planet, you get back to where you started; thus probably they recognize the spherical form of the planet).
There will be myths about why the world is like that; for example, they could assume that originally, all the planet was in light, but then a god turned evil and created the creatures of the dark, which ate away the light on half of the planet. They are constantly eating, but as they are eating the light on one side of the planet, behind their back the sun brings back the light on the other side.
There will probably also be myths about people who accidentally got into the dark side, and fought some of the mythical evil creatures living there, either successfully (for example with help of magic, or with help of the sun god) or unsuccessfully (the latter would serve as a warning to people never to enter the dark side).
Moreover, since the people are constantly migrating, there will likely be conflicts when two groups migrate to the same place. Such conflicts are likely driving history, and when they are sufficiently long ago, larger conflicts also will pass into myths, the stories of battles will get enriched with concrete ways the gods or mythical creatures intervened into the battles and cause one side to win (the side who tells the story is, of course, always the good side; this is true for both sides).
Note also that myths evolve; as they are told from generation to generation, they get enriched, put into relation to each other, get adapted to fit newer experiences, elements from one myth may enter another one, some myths may be merged, others may split into several different myths.
From the set of myths will eventually emerge a religion, and the religion will in turn again reshape the myths to fit. A religion always is also justifying the power structures in the groups following that religion. Therefore there will also be myths that relate to the power structures, that explain why those in power are in power, and why they have to be in power.
Over time, there will also be a large body of knowledge about which plants are edible and which are poisonous, which animals can be hunted, etc. Also this knowledge will likely be connected with myths. There might for example a plant that is extraordinarily poisonous. Then there may be a myth that explains how that plant became so poisonous.
In short, the key to believable myths is that they are born out of the experiences of the people, are made to appeal them, and in turn again shape their behaviour, therefore they cannot be invented in isolation, but only by considering how the people experience the world, what they fear, what they hope, and what they want to understand.
Often, myths are created by people in order to explain things that they don't know or understand in terms of things that they do know or understand. In most cultures, this involves ascribing some form of personality and intelligence to observed natural phenomena, and then trying to relate that back to their everyday existence.
Thus, "the river" becomes "the river that makes our crops grow", which becomes "Mother River, who feeds us from her breast". The question "if she loves us, why does Mother River flood and destroy our village every year?" leads to "why do other women in our lives become mean from time to time?" leads to "oh, she must be on her period".
From here, there's usually added a thousand years of people sitting around the campfire being bored, and trying to outdo each other with their stories. Anecdotes, gossip, bragging, propaganda, and outright slander from these people's lives get run through the filter of the existing mythic framework, and they come out as a soap opera about the lives of gods and heroes.
I would start with Jung's archetypes of human myths. From wikpedia:
"Jung described archetypal events: birth, death, separation from parents, initiation, marriage, the union of opposites; archetypal figures: great mother, father, child, devil, god, wise old man, wise old woman, the trickster, the hero; and archetypal motifs: the apocalypse, the deluge, the creation. Although the number of archetypes is limitless, there are a few particularly notable, recurring archetypal images, "the chief among them being" (according to Jung) "the shadow, the wise old man, the child, the mother ... and her counterpart, the maiden, and lastly the anima in man and the animus in woman".[9] Alternatively he would speak of "the emergence of certain definite archetypes ... the shadow, the animal, the wise old man, the anima, the animus, the mother, the child"."
And:
""These images must be thought of as lacking in solid content, hence as unconscious. They only acquire solidity, influence, and eventual consciousness in the encounter with empirical facts."[5]"
You don't have to actually subscribe to this analysis to find it useful as a framework for idea generation for your myths. If you consider the collective subconscious of the blimp people, and factors like - how does their theory of mind differ from ours - you might be able to come up with a revised list of archetypes for the blimp people and then use them to construct myths.
In Watership Down for example, which is a seminal example of Xenofiction, the primary archetypal motif for the rabbit protagonists is the trickster (embodied by the invented myths of el-ahrahrah, prince of the rabbits), whereas in human mythology the trickster is less important. This simple difference serves to create an endearing and effective rabbit mythos that feels quite alien.
The other advantageous aspect of adopting Jungs archetypes as a guide is that the literature that uses Jungs framework to deconstruct human myths is very rich.
By way of example, since the world is mostly land and rain is revered, the human 'deluge' archetype would need to be replaced by its polar opposite. Another suggestion is to try and specify the anima and animus - the archetypes of ideal male and female form.
Lastly I would also say that if the blimp people are not the only sapient race then the existence of other minds (now the people need two theories of mind one for themselves and one for the bats), is going to have to represent a huge difference between human and blimp-person myths, along with alterations to archetypes due to the long day/night cycle.
What I want to add is a little different to what other have contributed thus far.
Think of the story you are telling in your world, and tease out themes. Once you've decided which are the ones you want to emphasize, write a story (using some of the excellent references here) about that theme.
Once you've got the general plot (myths and legends are usually all plot with flat characters), imagine how that plot would have changed over time. I'm looking at a book on my shelf right now from 1893 called "Cinderella: 345 Variants". Different cultures in your world would interpret and re-tell the same story. The older your world is, the less the variations on the myth would retain similarities to each other.
The important thing is (for your story at least, not the real world), the THEME of the myth should stay the same. Or at least similar but interpreted differently.
My favorite example is "The Tale of the Three Brothers" from Harry Potter and the Deathly Hallows. The theme of the Harry Potter books is death. JK Rowling wrote other fairy tales, but this one, where the titular three brothers literally meet death, is the one she included in the book and integrated into the plot.
One of the most important aspects I see in myth development is that, from time to time, an individual is going to act on the assumption that myth is true. If the result is undesirable, statistically speaking, the myth is not going to prosper. As a result, I have found most useful myths provide enough useful value for driving a culture forward to make up for the occasional time where it proves to be a falsehood.
An excellent example is the myth of Icarus. Icarus flew too close to the sun on his makeshift wings, melted the wax, and fell. Every now and then someone might actually take this seriously, and fear getting too close to the sun. On its own, that myth would do nothing but pin people on the ground. However, it also tells an important tale of reaching too far towards glory, and paying the price. The value of this tale more than offsets the cost of people taking it literally. Good myths should all do this.
