In a Nutshell
Avocados are toxic to almost all animals (including cats and dogs). Humans are a rare exception. It is the only fruit to contain persin, a fatty acid, which, when eaten by animals causes vomiting, diarrhea, and other nasty symptoms. Consumption of large quantities can cause death within twelve hours.
The Whole Bushel
Avocados are berries (fleshy fruits coming from a single ovary). Interestingly, this broad definition of a berry means that bananas, pumpkin, tomatoes, watermelon, and coffee are also berries (you can tell that to the next person who tries to argue that tomatoes are vegetables). Curiously this also excludes strawberries as berries.
Eighty percent of modern avocados originate from one “mother” tree which was patented by mailman Rudolph Hass from California in 1935. The tree survived until 2002 when it died of root rot. Unfortunately Hass only made $5,000 in his lifetime from his patent on the tree because his partner sold cultivars to anyone who wanted to buy them. Subsequently Hass spent the remainder of his life working for the California Mail Service.
Avocado also has an interesting characteristic: it is the only berry with no living animal large enough to spread it through consumption and release as dung. This has led scientists to believe that it co-evolved with prehistoric megafauna that were large enough to eat the fruit whole. The megafauna went extinct but the avocado remained as an unusual monument to an unknown dinosaur.