If you're asking if a horse CAN mate with a woman, the answer is yes. There's no 'what if' about it - horses and dogs are the two animals most often associated with bestiality. In other words, people who have sex with animals most often have sex with horses or dogs. There's plenty of disgusting video evidence of this on the internet.
If you're asking if a horse can mate with a woman and produce offspring, the answer is no. For two different species to breed, they must be closely related (like horses and donkeys, or lions and tigers) - usually within the same genus, and at least within the same family. Horses and humans are far too different to be able to breed.