The two men, aged in their twenties, were caught red-handed on Friday night at the stables, near the town of Saint-Nazaire, western France.
The stable's owners had grown suspicious over the last ten months, after repeatedly arriving to work on Monday morning and finding signs that someone had forced entry into the property.
At times, some of the horses had even apparently been let loose to run around, reported the Presse Ocean publication.
In a bid to find out what was going on the woman then set up cameras at the entrance, which sure enough revealed that two men had been sneaking into the stables in the dead of night.
While the CCTV images showed the two mean approaching the stallions' paddocks, they did not reveal exactly what they were up to.
So together with some of her friends, the woman decided to hold a stakeout out in the stables on Friday to see what exactly was happening each weekend.
According to Presse Ocean, the group hid as the two men entered the stables and then proceeded to have sexual intercourse in front of the horses.
Then it is believed at least one of the men began performing a sex act on the horse before he was confronted by the owners.
They owners discovered that the pair had been engaging in sexual acts with the horse, including penetrative sex for the last 10 months.
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The police were called and both men have since admitted their crimes, and will face animal abuse charges.
Bestiality has been illegal in France since 2004. The crime carries a maximum prison sentence of two years and a €30,000 fine.