Where do the nostrils lead?
They have nostrils that lead to internal nostrils inside the mouth that lead to their lungs. When larvae they have gills which filter the water into the body and put into the plentiful blood vessels inside the gills. When wet or moist, most amphibians can get some oxygen into their body through their skin.
A Shetland pony's nostrils are in proportion to its face, so although they have smaller nostrils than, say, a shire horse, nostrils on a Shetland pony do not usually look particularly small for that pony. I have two miniature Shetland ponies, and from memory I would say their nostrils are about 1-2 inches big.