The XV Century Cerberus Saint appeared when Hakurei used his "Celestial Spirits Wave" attack to seal Hypnos with the other Saints of the Sixteenth century.
Another Cerberus Saint appeared in The Lost Canvas anime-only flashback of the XV Century Holy War. He was forming the last line of defense along with Altar Hakurei and Cancer Sage when Hades invaded and nearly destroyed the Sanctuary. He was probably killed either by Hades, Hypnos or Thanatos' attacks. His broken mask was later seen when Sage mourned the death of his companions.
As Hakurei used his "Celestial Spirits Wave" to summon his fallen companions in the previous Holy War during the battle with Hypnos, he is likely to be the same Saint.