Do they roll the vegetation in balls when eating? Does the stomach or intestines ball it up. Do they have a magical butt? Why are rabbit pellets (smart pills) so spherical.
I was just wondering the same thing about my guinea pig, and also deer. Human waste tends to look more or less extruded, reflecting the shape of the large intestine. But these other animals shit little very regular-shaped pellets of one size or another.
You just know they do this, but never wonder how.
The pellets are formed in the large intestine. There’s no great secret to it. The intestines of all animals move the mass along by a series of contraction waves in the gut walls. In the case of animals like rabbits and sheep a lot of water is extracted during the process and the mass becomes ever smaller. Because it is also very fibrous the contraction waves pinch the ever decreasing mass into pellets.
It’s probably easier to visualise if you look at the diagram.
Humans don’t do this because our food isn’t particularly fibrous and we aren’t very good at extracting water from faeces, so the mass remains semi-liquid.
As to why they are spherical, it’s because the sphere is the default shape formed when you roll something along a tube. Animals like horses and sheep also produce round pellets, but because they get jammed together in the rectum before being released they become distorted. Rabbits release their pellets continuously as they pass down the tract, so they never become squashed and remain round.
Nitpick: in the morning, rabbits poop clusters of soft tiny round poop (cecotropes). Then they eat them. They they poop firm fibrous round poop the rest of the day. Rabbits are weird.
The Master speaks. In short, it has to do with the colon, the rectum, and the sphincter working in concert to produce neatly-formed feces. Not so much with the diet of the animal - most pellet poopers are high-fiber herbivores, but many high-fiber herbivores have unformed, soft, or even near-liquid poop.
Thanks.
My cat pretty much poops spheres too, they’re just jammed up in a long string but you can clearly tell they’re spheres jammed up into a string.
Come to think of it, you can tell human poop is made of little rounded pellets jammed up too. But damn if I’m going to google for pics. 
My poop isn’t pelleted and never has been. See the Bristol Stool Scale - there’s lots of ways for us to poop.
But cat dung definitely isn’t a long string of spheres. The carnivore gut doesn’t work that way.
Never underestimate the power of the SDMB. You can find any kind of shit here.
You think that’s strange, believe it or not wombat poop is square (or cubical, roughly, to be more exact).
Best fact of the day. 
Yes. Yes, they do.
Now I’m thinking of the Trix rabbit with a magical butt.
Silly rabbit - shits are for kids!