Looks like this horse is laughing

The horse is actually reacting to the smell. It is called the flehmen Response. The flehmen response is a behavior in which an animal curls back its upper lip exposing its front teeth, inhales with the nostrils usually closed and then often holds this position for several seconds. It may be performed over a site or substance of particular interest to the animal (e.g. urine or faeces) or may be performed with the neck stretched and the head held high in the air. (source)

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