Without knowing the exact terms contained in the loan agreement it is not possible to specifically advise you, however the following is a rough guide and may be of some assistance:
A loan agreement is in effect just like any other legally binding contract: You are obliged to keep up your end of the bargain, and she is obliged to keep up hers.
You don’t say if the horse was being looked after by someone else, or if she was being left to fend for herself. If it is the former then unless you specifically contracted otherwise, that is not a ground for breaking the contract. If however the horse is not being tended to each day, as per the requirements of the contract then you do have legal redress. You can either seek specific implementation of the terms of the contract meaning that if you were successful you could force her to go up each day – to do this you would have to go to court, and there are no guarantees that the court would order this. Or you can cite a material breach of the terms by her, meaning that the breach of the terms of the contract is so fundamental, that the contract is no more. This means you should be able to take the horse back straight away. To go to court is costly, and quite frankly, would you really want someone like that being in charge of your horse. The latter option of removing the horse sounds, in my opinion, the best option.
Before you do anything, you should check the loan agreement for any clauses which deal with the return of the horse to you, because, as stated at the outset, this is a legally binding contract. If you have any concerns, you should seek the advice of a solicitor.
Having seen problems with loans in the past, there is definitely something to be said for watertight loan agreements, no matter how well the parties know each other.

Please note that this does not constitute legal advice and the writer will not be held liable for anything at all!
 

Hope this helps

If she has signed a document to say that she will tend the horse daily, then I would say she is legally obliged to do so.  For the horse's sake take her back immediately, there are plenty of good people out there who would love a horse to look after who would do a much better job.