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Large animals tend to be more vunerable to extinction than small animals, as there are smaller numbers of them, and they can't scavenge small pockets of food as effectively.

Burrow and den dwellers are acustomed to low oxygen levels. It's possible 6 ft beavers denned in more open areas, while their 2 ft cousins denned in a more familliar enclosed lodge.

I find it fascinating that the North American cows (bison) and the goats (deer and antelope) survived where the camel and horse did not.

I'm guessing here. Camel and horse are almost exclusively flatlanders - low altitude. Goats deer and antelope also dwell in mountains - high altitude. If there is a drop in oxygen levels the high altitude critters could move down hill to an area with higher partial pressure of oxygen. Flatlanders are stuck, no place to go to get enough air, they can't run far, think clearly, etc...