Shell is covered, somewhat. Material use will depend on area and type of creature. Scales are more likely to occur in deserts, and mountain creatures may even be made of stone. I'll explore these options a lot more when I do a perverse generator. This one is for tame, regular creatures, not the wild lovecraftian abominations of the chasms. When I work on those, there will be a lot of interesting creatures...
I can add tails as a "flavor" addition, because it doesn't really change the creature except to add some appearance. I have a few flavor/function body parts, like horns which add appearance but also add the ability to bash or gore, depending on it the horn is rounded like a ram or pointed like a bull. However, the creature is more likely to bite, punch, or scratch, so horns are mostly decoration.
Some creatures will be [COMMON_DOMESTIC]. All will be [PET] because let's face it, that's always cool, but some of the more tame creatures will be domestic. I've got a variable to decide if the creature is aggressive or not. Aggressive ones are more like wolves, and non-aggressive are more like dogs. One is easier to domesticate than the other, even though they're very similar. Some may be untrainable entirely, like very aggressive beasts that actively attack your fort.