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:
Mini vMac
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.
http://www.gryphel.com/c/minivmac/
SheepShaver
The most modern. Runs MacOS 7.5.2-9.0.4, emulates a Power PC based Macintosh.
https://sheepshaver.cebix.net/
Basilisk II
It lays in the middle of the previous two. Runs MacOS 7.x-8.1, emulates later color Motorola 68k based Macs.
Catakig
Virtual ][
Apple II emulator. Less popular than the others.
SWEET16
Apple II emulator. Less known that the others.
http://www.sheppyware.net/software-mac/sweet16/