Question: what kind of horses were used in the pony express
[{'answer': 'Morgans', 'provenance': [{'wikipedia_id': '43262', 'title': 'Pony Express', 'start_paragraph_id': 67, 'start_character': 73, 'end_paragraph_id': 67, 'end_character': 80, 'bleu_score': 1.0, 'section': 'Section::::Horses.\n'}]}, {'answer': 'thoroughbreds', 'provenance': [{'wikipedia_id': '43262', 'title': 'Pony Express', 'start_paragraph_id': 67, 'start_character': 85, 'end_paragraph_id': 67, 'end_character': 98, 'bleu_score': 1.0, 'section': 'Section::::Horses.\n'}]}, {'answer': 'Mustangs', 'provenance': [{'wikipedia_id': '43262', 'title': 'Pony Express', 'start_paragraph_id': 67, 'start_character': 154, 'end_paragraph_id': 67, 'end_character': 162, 'bleu_score': 1.0, 'section': 'Section::::Horses.\n'}]}, {'provenance': [{'wikipedia_id': '43262', 'title': 'Pony Express', 'start_paragraph_id': 67, 'start_character': 0, 'end_paragraph_id': 67, 'end_character': 229, 'bleu_score': 0.9517713210466091, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 10306870166259856333}, 'section': 'Section::::Horses.\n'}]}]