Voyage To The Prehistoric Planet (1965)

"In 2020, the moon has been colonized. Venus is next. three spaceships; Vega, Sirius and Capella are launched to explore the planet.

Two of the three spaceships meet either doom or mishap, leaving only the Sirius to land on Venus. The Astronauts go out to explore the planet and find a lush prehistoric world, similar to Earth in the Carboniferous period. (This movie predates Venus exploration probes in real life and therefore the speculation of a steamy living Venus was still very popular) Of course, they are soon attacked by prehistoric animals.

There does not seem to be any sign of intelligent life, but the astronauts keep gettng the feeling they are being watched. They survive their adventures with the help of a faithful robot, who of course, meets his doom at the end.

Eventually the astronauts return to Earth. .still with the feeling they have been watched all the time they were there, but never see anyone. The end gives a hint of who was watching them."