A little after 10:00 p.m., people discovered Dolly splashing around in their swimming pool.
While it's not clear how the horse got into the pool, the rescue team knew they had to get Dolly out - and quickly.
"It was around minus 1 [Celsius]," Rachel House, press officer for Hertfordshire County Council, told The Dodo. That's about 30 degrees Fahrenheit. In other words, it was freezing, and not the best weather to be paddling around in a pool.
If the rescue team didn't get Dolly out, she'd probably die from hypothermia.
A group of firefighters rushed to Dolly's aid, and spent two and half hours rescuing her.