I used to watch the Saturday Supercade cartoon with an older friend many years ago in the 80s. I remembered the Pitfall segment of the show had the main character Pitfall Harry, but also introduced his niece, Rhonda, and their pet lion named Quickclaw. As far as I know, these characters were made up for the show, presumably to give Harry someone else to interact with. These characters then later appeared in Super Pitfall as characters you needed to rescue to beat the game. I didn't know this before making this post (I had to look up Rhonda's name), but they also appeared in Pitfall 2: The Lost Caverns - which I've never played so I didn't know about that before now.
That made me wonder if anything else had ever appeared in a non-canon source and then later appeared in the video game the source was based on. The only other thing I can think of like this off the top of my head is that Scratch and Grounder from the Sonic the Hedgehog cartoon appeared in Dr. Robotnik's Mean Bean Machine.