Well it is called a "mounting stool"

Sorry but one line in this report really struck me as rather funny (my emphasis):

"A man has been jailed for three years after being caught having sex with a horse.

Leeroy Le Gallais, 46, broke into the animal's stable on two separate occasions to perform sex acts on the terrified animal.

During his first attack he used a bucket to stand behind the horse, called Calico, but was caught after leaving his underwear at the scene.

He was given a three-year probation order, but just months later returned to have sex with the same horse at the Castel Stable in Guernsey.

On the night of the second attack, on April 25 this year, Calico's owner Michael Wortley checked on the animal in his stable at 6.30pm.

The 20-year-old bay gelding was covered with a blanket but when Mr Wortley returned in the morning the blanket was on the floor.

A mounting stool that was left outside the stable had been taken inside and police immediately suspected Le Gallais was responsible. "