Scorpios

[Stefan Wirtz_2]

In the night sky, the constellations of the snake (Serpens) and the snake holder (Ophiuchus) are directly above the constellation Scorpius.

Fine, and Gemini is next to Cancer.
Both facts are no explanations, why most Zodiacs show a scorpion at Scorpius position, but some Zodiacs use all kinds of dragons instead, between Libra and Sagittarius. 
This may have a more cultural background, rooting back in ancient Greek or Greek-influenced understanding of Ophiouchos being the more relevant constellation here.
Transferred by the Romans to Britain/Bretagne, where the „dragon“ survived, and spreaded to Byzantine/Slavic circles also.