It is likely a name chosen by the horse's owner or breeder, and might be based on:
* A made-up name: Many racehorses have unique names that are not derived from any known language.
* A name from another language: The name could be from a language like French, Arabic, or even an invented language.
* A personal meaning: It might be a name with a personal meaning to the owner, perhaps relating to a family member, place, or event.
Without more context, it's impossible to determine the meaning of "Dunaden."