Garlic
Source: https://cdn-prod.medicalnewstoday.com
One of the most common ingredients in adding flavor to food, garlic is the most toxic of all Allium plants, which contain a variety of organosulfoxides, particularly S-alk(en)ylcysteine sulfoxides, which when eaten, is converted to sulfur-containing organic compounds. This can deteriorate the red blood cells in dogs, leading to anemia, which has symptoms like elevated heart rate, weakness, and pale gums.