In Latin, Венера is Venus. A similar word "corpus" is translated as "корпус", not as "корпора" (or "корпор" if we take its gender into account). Why is then Venus not Венус?
Imparisyllabic Latin words (meaning words having an extra syllable in genitive compared to nominative) are usually cited in their genitive form, as it's usually a more accurate representation of the word's etymology.
Church Slavonic authors chose to use Latin genitive stems for all Church Slavonic forms, including the nominative.
That's why we have Venus (gen. Veneris) > Венера, Cicero (gen. Ciceronis) > Цицерон, cliens (gen. clientis) > клиент etc.