Was there a time in Earth's history when some animal life existed on land but humans could not have breathed?
There isn't evidence that the worldwide air has ever been toxic compared to today since animals came to land 530+ million years ago. 

At some times in history, there has been lots of mercury vapor and perhaps other poisons in the atmosphere, for example recent research indicates that the late dinosaurs had at least 10 times as much environmental mercury from the Deccan traps eruptions which give most of southern india it's current geology. That said, the air would have been breathable, but some apex predators would have had mercury poisoning from their diets. Imprecise studies have found mercury levels in sediment go up to 1000 ppb for hundreds of years, compared to perhaps 100 ppb as they are today... other studies may find that there were higher mercury spikes, up to 30 times those of today perhaps. https://www.researchgate.net/figure/Temporal-correlation-of-mercury-records-with-other-indicators-of-Large-Igneous-Province_fig15_328051765

For oxygen, ever since the first insects emerged on land 530 million years ago, the oxygen levels have been safe for humans. https://en.wikipedia.org/wiki/Timeline_of_the_evolutionary_history_of_life