Before the current OS for Apple systems (MacOS X), there were earliest OS versions that were simply know as MacOS. MacOS and MacOS X are not that compatible, so if you need to run old MacOS programs or files then you need an MacOS emulator.
This is an extensive list of my known Macintosh system emulators:
The most classic, and also the one that supports MacOS earliest versions (1.1-7.5.5). It emulates a black and white Motorola 68K processor based Mac Plus.
The most modern. Runs MacOS 7.5.2-9.0.4, emulates a Power PC based Macintosh.
It lays in the middle of the previous two. Runs MacOS 7.x-8.1, emulates later color Motorola 68k based Macs.
Apple II emulator. Less popular than the others.
Apple II emulator. Less known that the others.