Onions are on the bad food list for most pets. The vegetables may be tasty additions to human food, but for animals, the chemicals in the vegetable can be toxic. These compounds can cause damage to red blood cells, resulting in anemia, as well as gastroenteritis. It can take a few days for the effects to become noticeable, so you may not realize that your pet has even ingested onions. Any plant from the same family is dangerous, including garlic, chives, and leeks.