For a horse, that's not so unpleasant in itself - their legs are able to 'lock' in place. Some horses will voluntarily choose to be on their feet for that amount of time anyway (not that I've watched any individual for long enough to personally vouch for this!) and are able to sleep standing up too. This is common to all equids, these Przewalski's horses included. Being stuck in a shipping container with no view for 30 hours though, that's another thing entirely...
https://en.wikipedia.org/wiki/Stay_apparatus