I think there is so much more to this question than people are considering on this thread. XD Though, I'm sure most of the answers are meant to be read in a light hearted way.
I'll say, firstly, this is absolutely a discussion to have with your significant other. Only you and he know the entirety of your situation. Exactly how badly does he want them? Enough to get a divorce if he doesn't get them? I'm not suggesting that's the case, just tossing that out there. (: Does he like horses? Would he be alright shouldering some of that commitment?
In terms of cost... unless you are purchasing a GP jumper or a World Class reiner (or something comparable for other disciplines xD) the cost of a horse will never amount to an even semi-spoiled (guilty xD) child. Someone mentioned not having to get kids shoes every six weeks after the college comment. I'll just say, I don't know many people who spent $100,000+ on a horse's feets xD
Sure, you could give a child the bare minimum--clothes, food, shelter. But who really does that? There's toys, there's field trips, there's birthdays, there's extra curricular activities. The list goes on.
As said before... A horse can be sold or leased. You can't sell or lease a child. XD
On the other hand... There's the idea of "time". You could feasibly buy a horse for many more years. In your forties, fifties, maybe even into your sixties or seventies. If you truly want a child, that's a decision you have to make /before/ your forties. So I guess that's an argument to be made for the "pro-child" side. A horse can always come later, once the kids are out of the house.
All of this makes me wonder... Does it really have to be one or the other? Both are time consuming and pricy... Both are very rewarding. Maybe you could find a way to make both work, after a conversation with DH.
My final statement to this discussion is this; people shouldn't be looked down upon either way. There are some people who are practically born wanting to have children. They spend their childhoods playing with dolls and emulating "house". I have friends like this. That's fantastic.
On the other hand, there are those of us who have never aspired to become a parent. I'm one of those xD I know that I'm too selfish to raise a child, and have everyone involved (my future husband, a potential child, and myself) all be happy. It doesn't make anyone less or more. It's just the way things are.
The sensitivity of the internet baffles me.