A pas de deux is an equestrian performance using two horses. The horses perform dressage

Dressage

Dressage is a competitive equestrian sport, defined by the International Equestrian Federation as "the highest expression of horse training." Competitions are held at all levels from amateur to the World Equestrian Games...

 movements, usually mirroring each other. It is seen not only in displays and musical freestyles, but also in performances by the Spanish Riding School

Spanish Riding School

The Spanish Riding School of Vienna, Austria, is a traditional riding school for Lipizzan horses, which perform in the Winter Riding School in the Hofburg...

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Two horses do exactly the same movements next to or across from each other. The term means "steps of two," Hence the mirrored moves.

A pas de deux is performed to festive music.