"Digital Devil Saga" has been appearing in the games' titles since the very first one, preceding the "Shin Megami Tensei" label, so I wouldn't exactly call it a spin-off.

Megami Tensei Gaiden: Last Bible and Last Bible II were released on the GameBoy in '92 and '93. Then they were rereleased for the GameBoy Color in '99, but only the first one was brought to America--apparently handled by the same team that gave Persona its abominable translation, since it was renamed Revelation: The Demon Slayer.