Mojave guest OS on Yosemite Host via Parallels Desktop 10
Here are the visualizers that I know about. 

Parallels:
Full version
http://www.parallels.com/en/products/desktop/
fyi: Free version of Parallels for individual use:
https://itunes.apple.com/us/app/parallels-desktop-lite/id1085114709?mt=12
Free version, if still available, will only work with the most current version of macOS.

VMware Fusion
With VMware Fusion, run the most demanding Mac and Windows applications side-by-side at maximum speeds without rebooting
http://www.vmware.com/products/fusion/

VirtualBox
VirtualBox is free, but you get what you pay for. Works fine for non-macOS operating systems.  Nightmare to get to work for macOS.  Basically, you need to match a version of VirtualBox with a version of macOS that will work. 
"VirtualBox is a general-purpose full virtualizer for x86 hardware. " Runs Windows on Mac OS.
http://www.virtualbox.org/