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:
- They explain why the world is as it is
- They explain why you should behave in a certain way
- They pass on and explain history and give an identity to the people
- They are entertainment
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.