Our horses are used to dogs but I find the best thing to say in a very loud voice, if you see a person approaching with a dog, is ' Watch out my horse will kick your dog he's done it before'. It usually works. Non horsey people rarely realise horses are not bicycles and the rider can never have 100% control. Maybe you needed to get your strength up after your recent very serious illness and operation, before resuming horse riding. PS -your horse did not throw you, you had an emergency dismount. ;-)