Equus is a term used on Equellus.
Judging by its use by Lacerta ("By Equus, you're right."), it is likely that Equus is some form of deity or something of importance (TOS: "The Lost Warrior").
Galactica 1980 definition: When John Steadman asks if the Warriors know anything about these creatures, Dillon looks this up (1980: "Space Croppers")