I know, that some wizards created well known spells, like "Wingardium Leviosa", "Alohomora" or "Reparo". Wizards use them very often. So I think, if the levitation spell was invented by some person, maybe someone created "Avada Kedavra" like that? I couldn't find information about the creator.
Unknown, but its creation was before 1717.
Nothing is ever said about the inventor of Avada Kedavra, but it was invented before 1717, because it was classified as an Unforgivable Curse by the British Ministry of Magic then.
“19 The Cruciatus, Imperius, and Avada Kedavra curses were first classified as Unforgivable in 1717, with the strictest penalties attached to their use.”
- The Tales of Beedle the Bard
As the Avada Kedavra curse would have to exist before it could be classified as Unforgivable, it would therefore have been invented sometime before 1717.
J.K. Rowling never - up till so far - mentioned the creator or the first famous user of the spell.
She only mentioned in an interview, according to another answer on another question, that it's ancient Aramaic (see How did Avada Kedavra get its name?).
If you dive deeper into ancient Aramaic and the etymology of the word, it is the origin of 'abracadabra'. And this language, Aramaic, dates back to the 11th century BCE. Originally spoken and used by ancient Middle Eastern people known as the Aramaeans. It is close to Hebrew and other nearby languages. (Source: Brittanica Aramic Language)
But there is also some doubt if it really is Aramaic. Language evolves over time, especially ancient languages are hard to trace back and interpret the same over years. (Source: AramaicNT.org)
Conclusion: It is by the creator of the Harry Potter books never mentioned who in the fictional world is the originator. And in the real world there is a lot of speculation where it originally comes from and in which language and who.