Green Vomit. Green vomit is often caused by mucous in the pet's body. Conversely, if the dog has eaten green houseplants or grass, he is likely to throw up green vomit. Some houseplants can be poisonous and if you find that the leaves of such plants have been eaten, seek medical help quickly. Stomach Infections. If your dog's vomit contains more mucous and less food particles, it would mean that he has an infection of the intestinal tract or stomach.