There is little to no truth in this because the Persian army themselves had their own cavalry that was made up of horses and they used it quite a bit. They used this cavalry under Cyrus the Great and other kings to take over the ancient Near East and it was very unlikely, if they used camels, that they were scared of horses. Mostly because by the time that Cyrus the Great made the Persian Empire both horses and camels were domesticated so the Persians would have been able to train them.

Plus, horses are very fearless animals and were pushed to go anywhere the rider wanted to go in the ancient world. So based on the ancient principles of horsemanship described by Xenophon and the many references of horses in battle throughout ancient history, it was very unlikely that camels were scared of horses. This just goes to show us that we need to exercise our critical mind when reading Herodotus because he was not always right.