One of the most dangerous foods for dogs is onions. In fact, the ASPCA National Animal Poison Control Center receives just a handful of calls a year concerning this deadly plant. Although large amounts of onions can be harmful to dogs, the symptoms of onion toxicity may not appear for a couple of days. Onions are especially harmful to Japanese breeds. Nevertheless, there is still good news. By learning more about the dangers of onions, you can keep your dog safe from harm.