The horse "ended up doing his business on the floor" inside the Whitefield, Greater Manchester McDonald's location on Saturday.
Chistine McGrail later admitted she acted "rashly" when the drive-thru employees said she and her daughter could not ride their horses through the drive-thru, but could come inside.
"When she said: 'You can't go through the drive through but you can come into the store', I thought 'okay, we will'," McGrail said.
Her daughter, Olivia, went inside with her pony, Minnie, while McGrail and her stallion Dancer waited outside. Olivia stood with Minnie in line for several minutes, but police were only called after Minnie relieved herself on the floor.
"I think over the last couple years, we've been to that McDonalds at least a half a dozen times on the horses," McGrail said. "I didn't see anyone in there who was terrified. Everyone saw the funny side."
But the police didn't see it that way.
"The sight and smell of this caused obvious distress and upset to customers trying to eat, as well as staff members," police said in a statement.
McGrail was fined £90 for causing "alarm and distress" and was banned from returning.
Other riders in the neighborhood sided with McGrail, complaining over the inconsistency in the McDonald's policy.
"I've been through loads of times," said another rider. "There are quite a few riding schools and stables nearby. The staff have always served me."