I'm afraid I had to break the news that this was not the case, the horse must have escaped from a neighbors and found it's way to our backyard on it's own.
To this announcement the children immediately hatched a plan to keep it forever and named it Dusty. Again I was the bearer of bad news: the horse would certainly be missed by it's owner and would not be staying.
Such a mean mom.
It was a fun couple of hours though...
Haven't heard a word about owning a horse since.