I agree with the other comments that you need to do something, but I'm going to disagree with that you should jump to divorce as the first step. You haven't said what you have already tried, so I suggest:
1) telling her how you feel, in the form of "I feel X when you do Y". For example, "I feel unvalued when you spend more on your horse than we do on the mortgage. I feel scared for our kid's future when you prioritize your expensive horse over saving for his/her education. I feel trapped when you spend the money I make without deciding together how to spend it." It can be hard to know exactly how these actions make you feel unless you've practiced thinking about it, so you might want to write it down and revise it over a week or two. Also, depending on how your wife takes feedback, you might want to have discussions of just one at a time.
2) marriage counseling.
3) setting boundaries: "my standard of being treated is <...>" and take steps to "enforce" them. The easier levels are along the lines of "I want to be talked to respectfully, so I will leave the room when you do not, but when you are ready to talk respectfully, come and get me." I'm not sure how you communicate "I think our budget should look 25% house, 20% food/clothes, 10% retirement, 10% kids education, etc, which leaves $X for optional things like horse; if you need more than that you'll need to get a job" without being unilateral, though. But you have some financial values/boundaries that are being crossed and you need to communicate / enforce those.
4) It would be a bad sign if your wife didn't respond positively to any of the above. However, even in that case you could get counseling for yourself on how to respond healthily, and you are also likely to get insight into why your wife is behaving this way (the counselor might notice consistent signs of co-dependency, for instance).
5) Read pre-modern stories about how spouses handled toxic behavior. (The quasi-mythic ones that start off "There was once a woman in ... whose husband ...") I've read a few Japanese stories about wives that change the incentives for their husbands and they stop being drunkards and start being productive. (There's fewer stories the other way, but those exist, too.) Some of these stories are quite creative solutions; maybe something like that would work with your wife.
Don't just stay stuck and miserable, though. There are many ways to defeat the giants.