Beats me. There’s a category for diarrhea.

That's even stranger.

There is a scat tag.

Scat simply wasn't mainstream enough when they came up with the categories here, so it got left out. Prior to around 25 years ago, few people had ever seen the word, and would not have known what it meant. But, words like watersports and golden showers were in common use, even as far back as the 1960s. I think most people in online adult communities know what scat means now, but the average person is still unaware.

In any case, if you are a scat artist, it's important to use tags like scat to make your art findable. I often search for words like scat, poop, coprophilia, etc, but, I likely miss out on some art the isn't tagged properly. I love that it's gradually becoming more mainstream and easier to find.

Thank you for explaining all of that. Usually, when I search "scat" on here, it just shows a bunch of people's commission infos saying that they don't draw it etc...

You can tell the search to only see keyword tags, while ignoring the titles, filenames and description messages. I use this to search for scat stuff, and it doesn't find most of the "don't draw" stuff. Copy and paste it into the search box:
(REMOVE the space after the @ symbol after pasting it.)

@ keywords (scat | poop | coprophilia | coprophagia)

You can also go directly to the search page at:
https://www.furaffinity.net/search/

Once there, click on "Help", to see how to fine tune your searches. You can also click on "Advanced", and uncheck the "general" rating. That eliminates most of the results from people who don't draw scat. If you usually only want to find adult stuff, you can go to your site settings to make search default to how you like to use it.