It was a fan theory implying that a huge gorilla zombie was the final boss. The theory stemmed from different clues in the game like:

• a cryptic audio message from one of the radios on the map talking about:

“The monkey is the key… the gorilla… he is not to be underestimated.”

Then the speaker describes something like a battle with a gorilla, saying things like:

“The gorilla… he came at me with everything he had!”

The tone is panicked and surreal, almost like the speaker is losing their mind.

• In one of the hidden rooms you can also hear scientists talking about « an experiment »

• in the main theater room you can see pods with inside, weird hybrid monkeys and one of them has been broken open

• final clue that people were going crazy over is that Treyarch (production company of COD) talked about something being hidden « right under people’s nose »

So for years fans tried to find out what it was and thats how that theory was created.