Planet of Mystery — Terry Bisson

Marooned on Venus, Hall and Chang of the first Chinese-American expedition are surprised to find themselves breathing Earth-normal air; and alarmed to find themselves captured by beautiful Amazons mounted on foul-smelling Centaurs. "This can't be real!" Hall protests as he is dragged off to meet the Amazon Queen.
Chang is inclined to agree (even though the arrow in his shoulder feels genuine).
Then the robot rolls in ...