That is just plain easy and horrible. It will depend on the age of the person as well, but when older, I know that in most cases they are developed by dead moments, that is, moments of leisure. Those moments when you are doing absolutely nothing and you feel disturbed because you cannot find what to do, are the same moments when your mind begins to create activity and thoughts of all kinds are born, both positive and negative. Your brain gives these thoughts color, shape, etc … and sometimes it puts a face on them, thus sometimes embodying something based on an experience that is being experienced at a certain moment.
For example, a person who is afraid of the dark, who is locked in a room, without light, at night, can unconsciously create a character of great stature, with a dark face, with a long cape and with a low sound and dense, that may be one of your monsters.
Others, on the other hand, personify by groups the voices that we hear in our minds, because whether they believe it or not, those voices do exist. Doesn’t it sometimes happen to them that when they are thinking about something they feel as if they could listen to themselves? And that depending on what they are thinking, their voice could be more pleasant or unpleasant? Well, the same thing happens, for example when we are very tired, or very overwhelmed, we can imagine that in our mind there is a small group of people who are talking to us and annoying us all the time, these could be small monsters and the same happens with the creation of groups of characters that drive us, motivate us and lead us to do things well.