It should not resist 50 cal bullets, but an armor which could protect a creature from the US standard issue rifle, for example. Here are a couple of things the armor needs to do:
- Protect the creature from the bullets mentioned above. Take a look at @computercarguy's answer for more information on the bullets I am talking about.
- The armor could be created from a chemical in the air into a solid piece of armor, i.e. the atmosphere contains some needed component for the armor. This chemical does not need to be in the Earthen atmosphere, it just needs to be some sort of gas, I'll work out the details of how it got into the creature's atmosphere. Note this is more of set dressing than anything else.
- The armor should not be much heavier per cubic inch than steel. The creatures could take more, but they still need to be able to lift quite a bit around.
- The armor could theoretically be "fixed" within a month at the max, but a little longer would be fine. Not a hard requirement.
UPDATE: I am looking for two specific things the other answer is not looking for, repair time and a request for it to be based on things in the environment, a theoretical element which helps build the armor.