Many thanks for your patience. Generally, when a person places an order for something and pays a deposit they enter into a legally enforceable contract with the seller. It is implied that the seller has accepted the deposit as security and as proof that the buyer wants to proceed with the contract. So there would be a legally binding contract in place.
If she refuses to sell the horse and returns the deposit, the buyer can potentially pursue her for breach of contract. Whilst they cannot physically force her to sell the horse, they can seek compensation for any losses they have incurred as a result. For example, if they had to buy a horse of similar age, breed, fitness, etc elsewhere and had to pay a higher price than what she would have sold it for, it would be the difference in price.
Saying that, it is relatively rare for such matters to be taken to the courts. They may threaten anything they want but they can only get something out of her if they actually make a claim and win. So there is still a long process to go through. Even if they do and win, she would not be responsible for their legal costs as this will go in the small claims court where each party pays their own costs. So the risks are relatively low even if she is sued and the worst is she has to pay them some small compensation and only if they can show that they have actually suffered losses because of this.
I hope this has answered your query. I would be grateful if you could please take a second to leave a positive rating (3, 4 or 5 stars) as that is an important part of our process and recognises the time I have spent assisting you. If you need me to clarify anything before you go - please get back to me on here and I will assist further as best as I can. Thank you