<The Safe Haven is a refuge for horses. It currently houses 1416698 horses.
Here are three of the horses in here, who are enjoying long, happy days. You can stroke any one of them you want to win a surprise each day!>
You can sell horse to safe haven then you can get 400 equus on one horse.
If you have a unicorn you can get 2000 equus.
you can go to your office and click the safe haven
*BE AWARE that if you send a horse to the safe haven, there is no way to retrieve it. Make sure you really do not want the horse, because once you send it to the haven, you cannot get it back.
I get some info from howrse wiki