Ophiuchus

Ophiuchus the snake-holder, also known as Serpentarius, is a recurring symbol in Final Fantasy series. Though usually not counted as a Zodiac Sign, Ophiuchus is the thirteenth zodiacal constellation, meaning the that Sun seems to pass through it during its visible route through the heavens.

Final Fantasy IX
Ophiuchus is the final Stellazio coin collected by Queen Stella. After all the other twelve are discovered, Stella begins to suspect that their may be a final, secret one. It is found around the healing spring in Quan's Dwelling, in the same location the Scorpio coin was found.

Final Fantasy XII
The Esper Zodiark, aligned with the element of Dark, embodies the sign of Ophiuchus. It does not have a Job assigned to it in the International version.

Final Fantasy Tactics
Serpentarius the secret thirteenth Zodiac Stone, and is inhabited by the Lucavi demon Elidibus. Unlike the other signs of the Zodiac, it does not have a month assigned in the Ivalician calendar, and so has no effect on character affinity. The stone can only be obtained by going down into the final level of Midlight's Deep and defeating the optional boss Elidibus. Like its unusual nature, Elidibus is an unusual Lucavi in that he does not join the others in the plot to revive Ultima. Indeed, it is unknown if he has any affiliation with the other Lucavi.