Avocado

Because of persin – a fungicidal toxin produced by the plant for self-protection – avocados are highly poisonous to many organisms.

An excerpt from the article 21 facts about Avocado
It is poisonous to horses, cattle, goats, rabbits, ostriches, chickens, canaries, corrugated parakeets, and fish. Most persin is found in bark, seed, leaves, and rind, and a small amount is also found in the flesh. Persin is generally harmless to humans.