* Neigh: A loud, drawn-out sound used for communication, often to call out to other horses.
* Snort: A sharp, explosive sound made when a horse is startled or excited.
* Whinny: A softer, higher-pitched neigh used to express excitement or to call out to other horses.
* Nicker: A soft, gentle sound used to communicate affection or to call to a foal.
* Squeal: A high-pitched sound often made by horses when they are playing or excited.
If you heard a sound and think it might be a "horse hoot," it's likely one of the sounds described above.
Perhaps you're thinking of a similar sound made by another animal? Can you tell me more about the context of this sound?