For all Ethereal monsters, one must breed a quad-element monster (i.e Entbrat, Deedge, ect.) with a tri-element monster (i.e Bowgart, Clamble, ect.).

The Ethereal monsters are extremely rare and aren't bred every time. When a wait time of 36 hours (27 with an Enhanced Breeding Structure) appears, you have bred an Ethereal monster. Any other time means you didn't, get an Ethereal and must retry.

On Plant and Air Islands, breeding the respective quad-element monster with a T-Rox is the most efficient method due to the eight hour (or 6 with EBS) wait time for T-Roxes when unsuccessful. On Cold, Water, and Earth Islands, any tri-element monster will do since the T-Rox is not featured here.