Steve Deeming attended Isleham Horse Trials, near Ely, on 6 March last year with his 15-year-old daughter Harriet, who was taking part in her first BE100 open of the season.
While helping with her pony, Mr Deeming was approached by a man who claimed he had been bitten by their dog when leafleting the lorry park.
“We have two dogs who have come to hundreds of Pony Club and British Eventing (BE) events with no incident,” Mr Deeming told H&H.
“Like many people we often tie them to the vehicle while we are attending the horse, but they are never left unattended. We were unaware of the incident until we were informed.”
Mr Deeming said he offered his apologies to the man, who showed them a picture of a cut to the back of his knee which had been taken on his mobile phone, saying it was caused by their Weimaraner, Ingrid.
The Deemings, of Rotherham, were visited by police a few days later and summonsed to Cambridgeshire Magistrates Court on 15 June.
Following legal advice, Mr Deeming pleaded guilty to being the owner of a dog dangerously out of control in a public place, although he was not convinced his dog had bitten the complainant.
He was ordered to pay £500 and Ingrid must now be muzzled in public.
“I was told we could have been forced to have the dog put to sleep,” said Mr Deeming.
“My message to dog-owners is to be careful. I would not like this to happen to anyone else.”
Dogs are welcome at BE events but must be kept under control, a spokesman said.
“Organisers can decide to ban dogs, but it would be quite difficult because of the culture at events, so none do,” she said.
An Isleham spokesman said they had considered banning dogs in the wake of the incident, but felt it was impossible.
“We would end up with people not realising and bringing their dogs. We don’t want to have dogs left in vehicles,” the spokesman said.
A notice reminding that dogs must be kept under control will be displayed in the secretary’s tent, they added.
This news story was first published in the current issue of Horse & Hound (3 March, 2011)