When reading the Pottermore writing on Azkaban, I took away the following highlights (emphasis mine):
TL;DR - Did Ekrizdis create Dementors?
The fortress upon it was originally home to a little-known sorcerer who called himself Ekrizdis. Evidently extremely powerful, but of unknown nationality, Ekrizdis, who is believed to have been insane, was a practitioner of the worst kinds of Dark Arts.
Those who entered to investigate refused afterwards to talk of what they had found inside, but the least frightening part of it was that the place was infested with Dementors.
Others were afraid of what might happen to the Dementors infesting the building if they deprived them of their home. The creatures were already strong and impossible to kill; many feared a horrible revenge if they took away a habitat where they appeared to thrive. The very walls of the building seemed steeped in misery and pain, and the Dementors were determined to cling to it.
The fortress was therefore left abandoned for many years, a home to continually breeding Dementors.
Experts explained to him that the only reason the Dementors were (mostly) confined to the island was that they were being provided with a constant supply of souls on which to feed. If deprived of prisoners, they were likely to abandon the prison and head for the mainland.
My feeling is that it is possible that Ekrizdis created the Dementors. However, the way the text uses tense and wording it could go either way. It seems that if they were not created on the island that the vast majority of them were hanging around there. Is there any other evidence to confirm the Dementors were created by Ekrizdis rather than just "existing" and being drawn to the island?
Related, but does not touch on initial creation and possible link to Ekrizdis: How are Dementors created/born?
It would not be the first time a dark creature were created; Herpo the Foul created the first Basilisk. Although, a Basilisk is a physical creature, while a Dementor is more of spirit/embodiment.