First of all, you must be able to establish criteria for what a mythical world is, and looking up this concept, you will find out that a mythical world is created to organize experience, is based on human experience, contains the story of a saving power, illustrates a pattern in human life, and can be enacted in a ritual. With that in mind, I suggest you choose to answer Question #3 first to choose a basis to organize your mythical world, and create one that is either evolving (developing) or devolving (dying out). If evolving, then the population of humans and animals will be moving towards, say some better form or perfection; if devolving, they probably are moving towards extinction. Choose your creatures accordingly. Question #2 will be the next logical one to tackle. If you select "Big Bang," then your mythical world would have begun "out of nothing," and that would eliminate Question #4. However, should you choose dreams, words or poof, anyone of these would be fine, and then you would answer #4 with an explanation, e.g. "When the creator uttered the word 'poof,' her almighty energy resulted in a flurry of creatures large and small, winged and hoofed, furry and scaled." Finally, you would respond to question #1 and describe what your mythical world looks like. The foregoing is just a suggestion; you can give free rein to your imagination as suggested here by another person and you could reference comics, movies, video games, and like sources. Feel free to reach out if you need any more help. All the best!