In real life nothing breathes carbon monoxide, but it still binds to the things that are supposed to carry oxygen around the body to my understanding. Could a form of life be able to actually use carbon monoxide to survive in an area with an unusually large amount of carbon monoxide in the air & little O2? This life did not naturally evolve & effectively just came into existence randomly, so this doesn't need to have evolved just be chemically possible. These creatures also live in a very, very hot environment, with temperatures regularly going into the high 60s. (centigrade)
What you need is an organ that acts as a biological catalytic converter
And I love the idea. Your world is platinum-rich! Your life has evolved to not die when platinum levels get too high in the body. And startled astronauts discovered this particular creature had evolved a natural catalytic converter in its esophagus. The result is that the creature is really breathing carbon dioxide.
Which isn't a problem! Because the CO2 is moved by the blood stream to the skin surface where chlorophyll (simplistically) converts it to sugar and O2.
Your creature will be a very lovely green. Green, with teeth! And it might need a high surface area to body weight ratio. So I'm thinking big fat frog looking sucker.
Granted, I jumped to the conclusion of using an off-world reference. The idea is easily adaptable to an engineered terrestrial creature that some brilliant twenty-something thought would improve our dying Earth — right up until he discovers it breeds too fast and likes the taste of blood! Cue the standard 1950s horror movie theme music. Include a scene where the twenty-something is sobbing and saying something like, "I didn't know! I just wanted to make the world a better place!" and we're in like Flynn.
Yes, Kind Of
You can certainly design an organic metabolism where a creature does not need to breathe in oxygen and can subsist on an atmosphere of carbon monoxide. However, this is going to work weirdly, because carbon monoxide doesn't really work well in organic chemistry. It houses a rare triple bond and has a bond energy of 1072kJ/mol. Let's say you wanted to take the oxygen from the carbon monoxide. A human being needs about 16.9 mol of oxygen per day, so that's about 18mJ, or a mere 4333 Calories. The good news is that you can combine the carbon radicals with some spare water to make around half a kilo of sugar, and that will get you around 1900 Calories back. In fact, this is what plants do - they use carbon dioxide to make sugar, and given that carbon dioxide is a lot easier to use and you get twice the oxygen, it's actually efficient! ...Unlike our system.
Except all the numbers I just gave you are slightly wrong. Organic chemistry isn't perfect, which means that despite the fact that all this is possible, it will probably take a process more than 18mJ per day to split the oxygen from the carbon, and reshaping the carbon back into sugar, which, while theoretically should be free, will take some energy to direct as well, meaning that a human being with these processes might need an extra 3,000 Calories a day, or so, and it will look like a very modified version of the Calvin Cycle.
Why am I designing a system that just supplements oxygen usage when it's so difficult? Well, because oxygen is just that good in organic chemistry. (It should also be noted that extracting oxygen from water has a far lower bond energy and makes more sense, but you specified 'carbon monoxide' breather, so...)
If your beasties, whatever they are, have an aerobic metabolism then they're gonna need oxygen. You can certainly use CO as part of a metabolic process, eg. Carbon monoxide as an electron donor for the biological reduction of sulphate, and there are useful hydrogen-producing reactions when you combine it with water, but there aren't energy-producing reactions where it can substitute for oxygen.
Anaerobic things, maybe even as complex as plants (or at least algae)? Sure. Animals? Less plausible.
The biggest problem with carbon monoxide to animals is that it binds to hemoglobin with a much great affinity than oxygen. Chordates are the only animals seriously harmed by it Insects are relatively immune to it as are plants.
The biggest problem you have with a CO atmosphere is its ability to both oxidize and reduce. In a mixture with oxygen it will burn in fact the gas in stoves used to be a CO/H2 mixture(Syngas) and CO will burn on its own. On the other hand in a methane atmosphere it will reduce the methane into water (releasing carbon and heat). Early Earth had a methane nitrogen CO2 atmosphere with possible trace amounts of CO.
It is perfectly possible to have things living in an atmosphere with a high percentage of CO. If you want to use it for metabolism keep in mind that whatever is converted sugar fat etc you are only going to get about 20% of energy that you would get with an oxygen breather.
The OP has stated in comments that "it's a weird situation, effectively completely alien environments to us just kinda, started, existing on earth, with these life forms in them".
The problem is that if we're talking about terrestrial life-forms, you can't get there from here. Terrestrial life forms have evolved to rely upon oxygen metabolism, and carbon monoxide effectively gums up the works, specifically, sticking to cytochrome oxidase and preventing oxygen metabolism Once that happens, aerobic metabolism stops until the CO can be cleared, and the cells must fall back to anaerobic metabolism.
So, unless these life forms are suddenly able to metabolize CO - which won't just happen with a single mutation, or are somehow able to cope with high levels of metabolic wastes such as lactic acid or alcohol from anaerobic metabolism, which also won't just happen with a single mutation, you're out of luck. The likelihood of multiple mutations allowing this is vanishingly unlikely.
However, there is absolutely no reason why life forms which evolved on a world with high levels of CO could not have evolved to be able to make use of CO in their metabolism, to both produce and consume it. Perhaps these life-forms are of extraterrestrial origin, and are converting our environment to more closely match their own. There is no reason why life forms able to metabolize CO couldn't survive in the absence of CO, with just oxygen and carbon dioxide... they'd just produce their own carbon monoxide.
So, I'd say that these life forms "just appearing" is not so much terrestrial life-forms mutating as non-terrestrial life forms (which may happen to look like terrestrial life-forms) arriving.
Yes, but not as a direct replacement for oxygen.
Carbon monoxide is a strong reducing agent; it wants to donate electrons, not accept them, and it's quite flammable. However, CO gas exists in equilibrium with CO2 and carbon, and in the right temperature and pressure ranges it is very energetically favorable for CO to disproportionate, producing CO2 and C from 2 CO molecules. Thus, your creature can generate energy directly from the gas it inhales, with no need of food, as long as it has a way of dealing with the elemental carbon waste.
Assuming that we're staying as close as possible to Earthling biology (rather than, say, designing iron-based aliens that drink link metal carbonyls), then the creature could also uses hydrogenic fermentation and react the resulting hydrogen with carbon radicals to produce methane, which gets you a bit of extra energy, eliminates the waste carbon problem, and indirectly connects CO gas and food intake.