This is for Ilvermorny School of Witchcraft and Wizardry Prompt #9. I have chosen dementors. This reads more like an excerpt from a textbook but I'm pretty proud of it.
In the grand scheme of things, Dementors are fairly young considering how ancient other creatures are like the centaurs and unicorns. Even Boggarts were older than Dementors and had been around as long as civilization. The first Dementors didn't look like the cloaked, skeletal figures we know today. Instead, they were reminiscent of Will-o'-the-Wisps but rather than vapor they resembled small piles of muck that slithered among the ground like a rather fast slug if the slug grew pustules and became slathered with tar. They wriggled and populated like maggots anytime a disease occurred. For the sake of differentiation, we will call them dementites.
They first appeared in Mesopotamia where civilization began with agriculture. With closer quarters and nearby standing water meant a greater risk for disease and this also meant more filth, more decay, and more rot from the deceased. However, they never lasted long for once the disease had passed the Dementites would die out from lack of a place to thrive for their true hunger lay in the emotions of the people who were tormented from the pain of illness. As they slept or, in some cases, lay unconscious, plagued by fevers and fatigue and pain the Dementites would attach themselves to the victim's mouth like a leech, draining them of their life source. Many would die from this and their families would be unable to see the parasites as magic shielded them from No-Majs.
And then… the most infamous plague in all of non-magic and magic history.
The Black Death.
It began in China. Travelers and merchants unknowingly carried the disease with them through the silk road infecting every town and country along the way with the one exception being Poland. Poland had a high jewish population and therefore excellent hygiene as a result of their culture.
The disease spread and so did the dementites. They grew in numbers and attacked No-Maj and Wizard homes alike. That is when the Dementor as we know it today emerged. These creatures thrived by the thousands and spread through Europe, China, and the Middle East. Their presence grew so strong that some No-Majs began to see them in their true form.
With the final major outbreak ending in the 17th century, the Dementors died off. They might have disappeared for good if it weren't for the workings of Ekrizdis, a wizard so evil that he attracted the Dementors by the sheer levels of dark magic. Ekrizdis was the first resident of Azkaban and used his fortress to lure No-Maj sailors in where he would torture and kill them. For more information on dark wizards, read Dark Arts and the Wizards Who Pursued It.
Dementors did not appear in North America until the Columbian Exchange which resulted in the Deaths of 90% of the Native Populations. The Dementors were rounded up and banished by the remaining leaders of the Native tribes to an island in a lake in Canada. To find out more read A History of Magical North America.