11 Foods Your Pet Should NEVER eat By Sherry - Jun 8, 2016 3133 Share on Facebook Tweet on Twitter tweet 8. Onions Image: Courtesy of Xiaojun Deng via Flickr (CC by 2.0) Onions can cause gastrointestinal irritation that can lead to damage to the red blood cells in pets. Cats are more susceptible than dogs however toxicity is a concern for all pets. PREV9 of 12NEXT