Have been recalling some other animal types from the past. Workmen ducks were common for a while, Jack Weasal is still appearing even in season 16, there was an incidental girl in Sweetwater's class called Mary. She is a lamb. There is occassionally a goat male character with a goutee . Lydia Fox was a design from a viewer that was adapted into a character on the show although a fox is almost a dog. The Tibbles are um... whatever they are. Mrs. Tibble is kind of a moose or caribou or something but without antlers. There is an alligator teenager.

There are tons of incidental aardvarks in the series but they look a lot like bears with different nostrils.

The only ones that  are  harder to animate would be the ones that have mouths that are part of a muzzle where the jaw needs to animate such as a monkey or rat. Because the whole head needs to be redrawn with the mouth (sort of). It's not a significant difference in work now with some of the changes in technology. You can see the monkey mouths from the 3/4 back view so the mouth needs to keep animating for dialog even in rear view. It's negligible.

The amount of animation would be more significant if characters have lose clothing or long flowing hair such as Muffy's pigtails or DW's dress or floppy ears because they can't just be traced back or they look stiff. But those things make the show look better.