A variety of Earth species have hard, more or less rigid shells covering parts of or their entire bodies as a defense against predators. These work to a varying degree depending on the specific predator's hunting style, but they don't really present much difficulty to human firearms.

Ignore for a second the pesky issue of exactly which gradual steps would lead up to something like this; let's just say that whatever evolutionary pressure is needed is present in the creature's environment. Here, I am interested in the observable end result, not really how we got there. (Though if you can give a brief description of how we got there, I won't vote your answer down for that reason.)

• Could a creature reasonably evolve something resembling a bullet-proof vest, as a part of its own body (that is, one or more layers that, taken together, are resistant to penetration by bullets), without invoking magic?
• If it could, then what would that "vest" be like? (More specific questions to get you started: What would be the properties of the material that makes up it? How would it appear if you were to touch it? Given these properties, could it help the individual in some other way? What would be its weaknesses?)

I'm looking for something that hopefully significantly reduces the risk of death from a (not necessarily point blank or perfect aim, but hit) round fired from a real world circa 1850-1900 human firearm that would be managable and usable by a single human with appropriate training.