A ham

A ham is an object, person, word, or phrase in media that shows up once and becomes important later down the line.

The term comes from the 2019 Invader Zim movie, Enter the Florpus, where a literal ham was introduced into the story and later on was used in the final battle.

"Now pay close attention to that ham, cuz it's not the last time you're gonna see that ham in the story." ... "See? There's that ham again."

"God, I had no idea the cat was gonna be a ham. It makes so much sense now."