Ruth, you did the best thing in the situation you were in.
As Loz says, not best to coast - but at least it cuts the engine noise.
There is a riding & road safety test that all horse riders are encouraged to take - in fact in the BHS & Pony Club it is mandatory as part of the examination system. This takes the part of a written test, and also a practical test - for which it is advised candidates attend at LEAST 6 training sessions.
However, many do not take the test (Iwish our equine insurers would load premiums for those who didnt)
You will find that most of the time, a horse or pony if frightened - and gets out of control - will try to get away from any sudden noise.
C
footnote:
Having been mashed in a stable this morning by my daughters pony stallion, I could cheerfully throw him under a car
- but then three quarters of a ton of moving object, that was not a happy pony while the equine dentist was doing his job, had every right to try to escape!