Platform notes: RetroPie

Other than the official Raspberry Pi releases you can found on the download page, Pegasus is also from the RetroPie repositories. You can install it by two ways:

Note: you will need RetroPie version 4.3 (released on September 21, 2017) or later to install Pegasus through RetroPie.

From the setup menu

You can start the RetroPie setup script from the terminal (sudo ~/RetroPie-Setup/retropie_setup.sh) or from another already installed frontend. You will be greeted with a text-based interface. To install Pegasus, select Manage Packages → Manage experimental packages → pegasus-fe → Install from binary. The latest release will then be downloaded. You can also update your existing installation here.

retropie screenshot

After the installation, you can either call /opt/retropie/supplementary/pegasus-fe/pegasus-fe manually, or edit /opt/retropie/configs/all/autostart.sh to launch Pegasus on boot (instead of eg. ES2).

Using the Frontend Chooser

There's also a graphical tool for installing various frontend and setting them up to boot automatically.

retropie frontend chooser screenshot

It is available from HERE.