The 13th sign of the zodiac - Ophiuchus

Via Wikipedia

Ophiuchus is a large constellation located around the celestial equator between Aquila, Serpens and Hercules, northwest of the center of the Milky Way. The southern part lies between Scorpius to the west and Sagittarius to the east.

Of the 13 zodiacal constellations (constellations that contain the Sun during the course of the year), Ophiuchus is the only one not counted as an astrological sign.

Further info:
Ancients omitted the 13th sign of the zodiac