Dementors are the guards of Azkaban prison for the wizarding world. What are exactly the Dementors?
Are they like Inferi, dead bodies reanimated?
Dementors are non-beings,as mentioned above.The HP wiki says they might be related to Lethifolds.If you look up the translations to other languages,they're called demons:
Taiwan: 催狂魔 - cuīkuángmó - "drive-mad devil/demon"
Indonesian: "iblis" - "demon of death and happiness remover"
They are neither humanoid nor animals,they seem to be demons,maybe a magical manifestation of fear,despair and hopelessness,the counterpart to the partronus. The reason why they're permanent?I don't really know,but i assume it's because of all the terror caused by Voldemort during the time he "reigned" the magic world (and ofc the fear of his return in the time the books/movies are playing).
In JKR’s writing on Pottermore on the Boggart, it’s mentioned that Dementors are (like Boggarts) classified as non-beings - creatures that have never truly been alive, which have no equivalent known to the Muggle world and are likely formed and sustained by human emotions.
Like a poltergeist, a Boggart is not and never has been truly alive. It is one of the strange non-beings that populate the magical world, for which there is no equivalent in the Muggle realm. Boggarts can be made to disappear, but more Boggarts will inevitably arise to take their place. Like poltergeists and the more sinister Dementors, they seem to be generated and sustained by human emotions.
- Boggart (Pottermore)
As, due to their classification, they’ve never been alive, Dementors aren’t reanimated dead bodies, nor are they created from anything that was at one point in its existence alive. Instead, they seem to be spawned by conditions of despair and decay, as that’s when their numbers increase.