Reuters / Suzanne Plunkett
The 82-year-old Conservative peer, who was an MP until 2001, made the admission in an interview with Tatler magazine.
He said: "I went to stroke him and he started biting me."
"If you have a dog that turns, you just cannot risk it. So I took Kim's collar - a short of choker chain - and pulled it tight."
"Suddenly he went limp. I was devoted to Kim, but he'd obviously had some sort of mental breakdown."
It is not clear whether he means the dog died or ... something else. Business Insider is attempting to contact Heseltine for clarification and this story will be updated when we hear back.
Heseltine was Deputy Prime Minister from 1995 to 1997 under John Major, and a prominent figure in Margaret Thatcher's government before that.
The RSPCA have told the Daily Mail that they are "considering their response."