Muls as represented in the DSCS are pretty close to how they actually are, but there are a couple of changes. First off, it’s mūl, not mool, or mull, despite the information given on p. 21, DSCS. Secondly, they’re called muls because, well, they’re mules. They’re sterile crossbreeds of humans and dwarves (typically with a dwarven father and a human mother), bred as slaves, and extremely valuable—in part because giving birth to a mul often kills the mother.
The picture in the DSCS (below) does muls justice, but I’ve also included some other images of muls for reference.
These muls are exceptionally pretty. Most aren’t so well formed:
Here are some other pictures of muls: