You might find Planet Zoo on sites like G2A, Kinguin, or Eneba for $10–$15. These are not "free," but they are cheap. However, you must understand the risk.
How resellers get cheap keys:
The result: You pay $10, play for three months, then one day Steam removes the game from your library with the message: "Product key revoked by issuer." You lose your zoo, your progress, and your money. planet zoo steam key free
Steam frequently hosts Free Weekend events for popular games. During these periods, you can download and play the full version of Planet Zoo for free for 48–72 hours. You might find Planet Zoo on sites like
Planet Zoo frequently goes on sale for 75-80% off. The result: You pay $10, play for three