There is nothing wrong to be a professional jockey in order to teach people how to ride horses. In fact, it is a good deed. The Messenger of God encouraged horse riding and breeding. However, participating in horse races that include gambling is not permissible. Because gambling is unlawful and cooperating in unlawful actions is also unlawful.
Allah the Almighty commands:
“…Help one another in righteousness and in avoiding unlawful things. Do not help one another in sin and transgression…” (5:2)
“Oh you who believe! Intoxicants, gambling, idols, arrows for seeking the future or luck are all abominations, the works of the strayed. So, keep away from them that you may attain the true success. The strayed ones only desire to create grudge and hostility between you with intoxicants and gambling; and hinder you from the remembrance of Allah (Quran) and the prayers. Will you, now, desist?” (5:90-91)