That's what I'm trying for. I really didn't know what category it would be under, so I just did Screenplay. Does that work? Not like I can change it. Nothing else fits.

I'm going to write it as though it were for a 2D animated movie, because, well, I think 2D animated movies are awesome.

Any thoughts on what category an animated film script would go in? Or even a proper name for it? I've only looked at Comic book and Sitcom ways to write scripts, so I'm looking forward to trying other things.

Animation isn't a genre; it's a medium. It then branches off into three types of animation: stop-motion, two-dimensional cel, and computer.
Just put your film in the genre it belongs to. If it's a romance, label it a romance. If it's a bloody massacre with dancing skeletons and skulls, then label it as horror. That's how we should be doing it.
I'm doing an animation script, too, once I do enough research. It's a historical fantasy about Japanese internment.