one who eats alone, or who eats only one kind of food ...1625 obs.
from Greek µονοϕάγος (monofagos) that eats alone
FIRST DOCUMENTED USE
1625 - see Example below
"...A Monophagian or Monophague, one that eats his meat alone without companie, or eats only of one kinde of meat."
From: Ductor in Linguas, The Guide into Tongues,
By: John Minsheu 1625
• A New English Dictionary on Historical Principles,
James Murray, 1887-1933