I've never heard of them being used for horses, however my sister shows dogs and as I understand it from her, some not-so-reputable show dog owners will have their dog neutered and have the falsies used so that they appear to be intact.
According to my sister, it is usually done when the owner can not handle their aggressive males when in a show ring with females in heat.
Be strong and courageous, and do the work. Don't be afraid or discouraged by the size of the task, for the Lord God, my God, is with you. He will not fail you or forsake you.
1 Chronicles 28:20