CCTV caught the woman and her daughter taking her pet into the restaurant after they were told they had to tie it up.
Customers then watched on in horror as the animal went to the toilet on the floor.
Police were called and the woman was handed a fixed penalty notice fine for ‘causing alarm and distress’ to customers trying to eat their lunch.
The fast-food chain do not allow horse riders to use the drive-thru in the saddle due to health and safety reasons.
The policy was introduced over fears that safety of staff and customers could be put at risk if a horse is spooked by a car.