The epithet "four-eyed" may possibly contain a tentative coagulation of
the two dogs in one. The capacity of the two dogs to see both by day
(the sun) and by night (the moon) may have given the myth a slight start
into the direction of the two-headed Greek Cerberus. But there is the
alternate possibility that four-eyed is but a figure of speech for
"sharp-sighted," especially as I have shown elsewhere that the parallel
expression "to run with four feet" is a Vedic figure of speech for
"swift of foot."[17] Certainly the god Agni, "Fire," is once in the
_Rig-Veda_ (i. 31. 13) called "four-eyed," which can only mean
"sharp-sighted."