Shinme (or Jinme, sacred horse) is a term used to refer to a horse that is dedicated to a Japanese shrine or one used in rites and festivals. There is no rule about the kinds of horses to be chosen, and they are believed to be ridden by god (deities, spirits).
Since the Nara period, it has been customary to dedicate horses as a prayer to god. Dedicators range from ordinary people to members of the imperial family.
Since caring for horses can be a burden for small shrines and horses are expensive for dedicators, they were gradually replaced by 'ema' (wooden votive plaques, originally with a picture of a horse). In many cases, a life-size statue of horse is regarded as a shinme.
In article 26, volume 3 of the Engishiki (an ancient book for codes and procedures on national rites and prayers), it is written that a black horse was dedicated when people prayed for rain and a white horse was dedicated when they prayed for fair weather.
Occasionally, when many horses are used at festivals and other events, they are temporarily called shinme.
Shrines that keep shinme
Nikko Tosho-gu Shrine
Iwashimizu Hachiman-gu Shrine