The sign is Taurus, a black bull or ox, and in one somewhat convoluted interpretation it has a place in the Creation of the world.

In this explanation, God began forming the world in the month of Nissan, not Tishrei. Satan, prince of darkness, demanded that darkness be the first element created. Genesis 1:2.

God refused, declaring that the world must first be illumined. So God created light and called it Day.

Only then, as the second act of Creation, did God fashion darkness and call it Night.
The white ram of Nissan, the first month, represents the light.

The black ox, symbol of the dark, became the sign of the second month, Iyar.
The dark parts of mans life being illuminated with God’s light, exchanging the blessing with a curse.

The “shor (the ox)” (the left face of the Divine Chariot) represents the spiritual origin of the “animal soul” of man.
The month of Iyar is the month that man rectifies his animal soul, refines his innate character traits (each day of “omer counting“), as he prepares to receive the Torah in Sivan. 

Taurus- refusing to be confined to the natural state of nature. Taurus enable’s one’s frequent act of counting the Omer in the month of Iyar to sanctify to heal, to upgrade and to harmonize the earthly nature with God’s truth.

In Hebrew, the root ” shor ” also means to look or observe. Iyar is the month of introspection for the sake of self improvement. 
By: The house of the prophets.