"Whose horse is it?"
Translation:À qui est le cheval ?
"whose horse is it?" is asking about one horse that is pointed to (it).
in French: "à qui est le cheval ?" or "c'est le cheval de qui ?" correctly translate that question.
"C'est un cheval de qui ?" sounds really weird. At the very least it could mean that you are pointing not to the owner of the horse but to his parents or to the artist who painted the horse or sculpted it.