Some of the first polymorphic viruses had very advanced code translation engines, basically JITs before they were common. I wonder if any JIT concepts were invented there?
One example is the DAME which stands for Dark Avenger Mutation Engine. It could recompile DOS viruses into equivalent but different machine code on the fly, including the mutation engine itself.
Also check out season 1 of https://malicious.life/. S1E02 specifically covers The Dark Avenger, but the whole of S1 was awesome (and he rest too. But S1 especially).
Maybe I'm confusing DAME with later mutation engines that had more elaborate ways of transforming the decryptor and payload code. I know there were some pretty sophisticated JIT-type things in the latter part of the DOS virus era.
The same thing can be said about game/software cracking scene groups. I believe they were (still are?) at the forefront of practical reverse engineering techniques.
An extension of that is the emulation scene, but that tends to be done in public for the most part.
I'm not sure it qualifies as an "invention", but they definitely do some incredible work. They're able to pull off full API emulations for things like Steam, Origin and even emulate UWP to a degree that allows applications to be run outside of the traditional UWP system. Professional reversers often use tools developed by crackers - things like x64dbg, ImpRec, unpacking scripts, etc are indispensable.
If it weren't for this community I'd feel far less in control of my own computer.
One example is the DAME which stands for Dark Avenger Mutation Engine. It could recompile DOS viruses into equivalent but different machine code on the fly, including the mutation engine itself.