These guys are the soldiers from Carmen. They always creeped me out a little.

Their synchronised appearances and movements are an advantage of the animation format, it's a little unsettling. And look at those teeth!
The set here looks like it's made of cardboard. Not a big budget at the Springfield opera, I'm assuming.
This post is part of my "Every Simpsons Character Ever" series. For a list of my rules in this project, click here.