There are a lot of variants of malware, with each having their own unique traits. Then there is one that continuously change.
This kind of malware is called 'polymorphic malware'. From viruses, worms, bots, trojans, or keyloggers, their polymorphic variants can have different characteristics every time they run, like having different file names and types or encryption keys.
With their ever-changing characteristics, polymorphic malware can be unrecognizable to many detection techniques.