Horses do not breathe like other mammals—they only breathe through their
noses because they cannot breathe through their mouths. The equine
respiratory system functions by inhaling and exhaling through the nostrils.
This is thought to perhaps be an evolutionary feature to prevent debris,
such as dust and dirt, from entering the horses’ lungs as they run. Their
large size makes it so that horses are only able to inhale as their front
legs stride, but not as their legs land because their internal organs are
proportionately larger than other mammals’. Horses’ lungs would then
not be able to expand efficiently to allow oxygen in during landing due to
the force that would shift their internal organs.