The ROG Phone is a smartphone. Most smart phones let you save any mp3 song available online by touching and holding your finger on it, then selecting 'save.' Once the music file is stored on your library, tap-and-hold it and select 'assign' in the contextual menu.
The Asus ROG is an Android phone and there are plenty of free ringtone apps such as MP3 Cutter, Zedge and more.
Here are links to some of the free ringtones (Asus ROG-compatible) offered here at MartinsRingtones.com:
Free Horror MIDI Ringtones Free Tropical Ringtones Free Ringtones-a-gogo Free Techno Ringtones Listen To All Our Free Polyphonic Ringtones