In most European languages Venus is Venus or something similar. In Eastern and Southern Slavic though it is Venera. How did that become? I know in Italian it's Venere, but I wonder if there's a direct link and how it became Venera in the Slavic languages.
Venus is the Nominative, Veneris — the Genitive. In the East Slavic tradition, the Latin and Greek words with unequal stems in the Nominative and oblique cases are usually borrowed with the latter, longer one, e. g. Nom. Atlantis, Gen. Atlantidos, Dat. Atlantidi, Acc. Atlantida is borrowed as Атлантида/Atlantida. The final -a was added to make it declinable.
In Czech / Slovak, Venus is an exception: Venuše / Venuša (declined like Nataša); ... regularly it would have be: Venus (Venera) - o Veneře;
For other Graeco-Latin names, there are often two possibilities in the nominative singular:
Atlantis (Atlantida) - o Atlantidě;
Artemis (Artemida) - o Artemidě;
Najás (Najáda) - o Najádě;
Atlas (Atlant) - o Atlantovi;
Pallas (fem.) - o Palladě;
Pallas (masc.) - o Palladovi;
Paris - o Paridovi, Paridův soud;
Mars - o Martovi;
Russian: Атлас or Атлант - об Атланте; but Парис - о Парисе;