Avocados, although toxic and harmful to birds, horses, and cows, are less of a concern for dogs and cats. One potential harm to dogs and cats comes from swallowing the pit of the avocado and getting it stuck in the intestines. The average pet will be unable to pass an avocado pit and will need immediate surgery to remove the obstruction.
Another concern is the fat content of the avocado flesh. Pets can develop a condition called "pancreatitis" from eating any food of high fat content including avocados, causing vomiting, diarrhea and stomach pain. Pancreatitis can require hospitalization. So, when avocado season comes around, keep track of those avocado pits, and keep that green avocado goodness to yourself!