Because a petrified T-Rex is considered a fossil, a 400-million-year-old fish preserved in rock is also a fossil and a preserved footprint from a dinosaur is a fossil, the cow is the only one that is not a fossil because it does not remain or an impression of a prehistorical, organism because cows still exist today
Definition of a fossil: the remains or impression of a prehistoric organism preserved in petrified form or as a mold or cast in rock.
