Asthma attack occurring while horse riding only rarely, with no warning, is there a way other than pre-treating to prevent them, as i ride every day?

Asthma: It might be the area where you are riding. Pollen, dust and other allergens are typical causes for asthma. Even humidity might trigger it. Please evaluate your triggering factors and try to avoid them. Prevention is the best treatment. Try to change your track and see if it helps. Good luck.

See An Allergist: You can be tested to confirm allergy to horse and if you are allergic, you can receive immunotherapy which will eventually make you not allergic to horses.