Yoda the monkey
There was a huge chance that Yoda was actually going to be played by a monkey in a Yoda costume and not the puppet we know now. Just think about that, they were going to dress a monkey up in a weird-looking mask and have it trained to walk around and interact with actors.
Everything was set up and ready to go, but one of the head crew members stepped in and said it would be a terrible idea. He had experience working with animals before and noted how much of a headache it was to direct them. After listening to his advice, Lucas decided to go another route.