Can my large creatures have metal scales?

Question

In my world metal is difficult for humans to obtain, it cannot be found mining, at least not in any useful form.

Instead, to obtain metal, you have to slay large and very dangerous creatures covered in metal scales. The metal scales make for great armor thus the creatures are very difficult to slay.

The metal creatures range in size from roughly a cow to an elephant.

The metal creatures need to obtain their metal in such a way that a human cannot just obtain metal in the same way, thus circumventing having to slay the metal creatures.

Is it possible for the creatures to have metal scales?
I would appreciate examples from nature of naturally occurring metals in animals, if there are any.

If it is relevant to the question:
The society is feudal with the nobility owning almost all of the metal, it is illegal for someone who is not noble to own metal. Most tools are stone age but with better craftsmanship.

Edit:
When referring to metals I mean the metals that are traditionally used to forge tools and weapons, such as iron, copper, tin etc

Answer 1

Metal Madness:

Life is biochemically capable of producing metals. Magnetotactic bacteria and Gold-depositing microorganisms are a couple of examples. But as THIS excellent question examines, there is little practical benefit to organisms to do so. It is extremely unfavorable to do so. Metals generally will spontaneously oxidize to various oxides, and to have these metals on the outside surface of an organism means they would be unable to take any biochemical benefit from these reactions. Unless your organisms live in a reducing environment, I just can't see the benefit.

Animals have very successfully produced physical protection that doesn't require such means - like hide and bony carapace. Metals are not stronger than bone, and may have distinct disadvantages. So why would your organisms have such things?

• I suppose it could represent some kind of electrical attack or defense mechanism. Given small gold bio-plated wires, an organism with gold spines might be able to create shocks to fend off predators or stun prey in a manner like an electric eel, but on land. This assumes small amounts of metals are produced by organisms, and that there would be an abundance of solubilized gold in the environment. Gold does not appreciably oxidize, so it would be stable once produced. But this doesn't get you a metal hide, and gold is not a great metal for tools. It is soft and would be a very poor metal for armor.
• Aluminum is not a part of hardly any biochemistry, it's even more energetically unfavorable than other metals, and it's generally very difficult to solubilize in any appreciable way. But if there were some biochemical extremophile in your world producing toxic levels of soluble aluminum compounds, then animals MIGHT have symbiotic bacteria precipitating metallic aluminum in the skin of these animals as a kind of waste removal. Aluminum rapidly forms a protective oxide layer (and is thus stable) and can be hard while extremely light. So while I can't QUITE see how this would work, there would at least be a sort of justification. Aluminum would be a very difficult metal to work in a pre-industrial society, so the applications would also be limited.

As a bit of a frame challenge, if you DID have metallic-armored animals, then people are people and would immediately farm them. Mining would be replaced by agriculture as tin cows were pastured near the best mineral deposits. Iron rhinos might be hard (pardon the pun) to domesticate, but people would quickly figure out a way. So metals would be abundant in your society because people would be able to raise them. In fact, it would advance metallurgy at a much earlier stage in human civilization. But all that metal in animals may discourage people from looking for alternate sources of metal, since it OBVIOUSLY comes from cows and rhinos.

A lot of the metals that ARE more stable oxidatively are also very toxic, so your humans may not do well in environments where your animals live. This could provide a reason they don't get domesticated more often.

If you do have iron scales, I might suggest something more like chromed or stainless steel (chromium) or galvanized steel (zinc), where the thin outer coating of another metal protects the surface from rust.