According to the ASPCA, garlic is a no-no when it comes to our feline friends. While it seems that animals can digest small amounts of garlic powder, larger amounts can make them anemic and cause gastrointestinal problems. In the case of garlic, the culprit is the chemical compound thiosulfate, which is naturally occurring and difficult for dogs and cats to process. As bad as garlic is, though, it's nothing when compared to the amount of thiosulfate found in the next item on our list, onions.