The C14 will undergo radioactive decay, and after 5730 years, half of it will be gone. So, if we find such a body, the amount of C14 in it will tell us how long ago it was alive. The method doesn't work on things which didn't get their carbon from the air.
This leaves out aquatic creatures, since their carbon might (for example) come from dissolved carbonate rock.
The disappearance must take place at a known and measurable rate.
Carbon dating is only accurate back a few thousand years.
So if scientists believe that a creature lived millions of years ago, then they would need to date it another way. They assume dinosaurs lived millions of years ago (instead of thousands of years ago like the bible says).
This date did not fit the preconceived notion that dinosaurs lived millions of years ago. They do this many times, using a different dating method each time.
The results can be as much as 150 million years different from each other! They then pick the date they like best, based upon their preconceived notion of how old their theory says the fossil should be (based upon the Geologic column).