This sounds like you should really do a cost-benefit analysis of this situation. Are the attorneys fees and costs of arguing about this going to outweigh the 50% that she is asking you to pay? If not, then you may want to retain counsel to argue about the language in the agreement and if she had the right to move the horses from the home before it sold. However, it seems like there is a requirement that the horse be sold as well. So, it seems like your timeframe here is limited and you need to think about how much you would spend to argue the point. Good luck to you.
This answer is for general informational purposes only and should not be relied upon for your particular case nor is it intended as legal advice. I have not reviewed your case nor have I met with you and the answer to this question does not in any manner whatsoever establish an attorney/client relationship.
The reason for her moving the horse may come into play if she brings the issue to court to enforce these expenses against you. If the divorce agreement specifies that the horse is to be maintained at the marital home, and it was only by her choice to move the horse, she will have to bear the expense of moving the horse and boarding the horse where she chose to move it to. Veterinary bills do not fall under the scope of boarding (food or shoeing), at least as you have defined it above. If the result of all of the Vet bills is because of a lack of adequate boarding at your home, then you could be liable.
The horse needs to be put on the market for sale this month, but based upon the agreement, your obligation to pay 50% boarding expenses starts September. There needs to be a good faith effort used to sell the horse as well, i.e. placing ads in appropriate places, Craigslist, etc.
I urge you to have an attorney review your divorce agreement to ensure you are not in violation, as my advise is only based upon what you have disclosed.
If you have been served with formal documentation associated with your question, there may be time limitations for you to file a response. I urge you to seek formal in person advise with an attorney to review your case and options thoroughly. This answer is based upon the information provided. If there is information ommitted, it may affect the outcome of your case.
This is not a general question; you are looking for specific legal advice. You should consult a local attorney for the specific advice you are seeking.
This is not legal advice. This response is provided for general information only, as a public service. It is not to be relied upon as legal advice and does not create an attorney/client relationship; nor is it an attempt to create an attorney/client relationship. Consult with local counsel in your jurisdiction about the specifics of your case, which is the only way to gain true meaningful legal guidance and/or representation.