Mescal Cactus is a very small round cactus that causes hallucinations. It grows in the Sandora Desert.
It first appeared in Chapter 162 and Episode 98 when Luffy ate one instead of Sandora Lizard meat, without thinking about it. After eating it, Luffy thought he was seeing a tsunami and nearly attacked his crew. Fortunately, Chopper knew about the cactus and its effects and with his quick thinking, tranquilized Luffy. The effect had worn off by the time Luffy woke up.
Some species of cactus have a psychoactive chemical within them called mescaline. This substance causes confusion of the senses (although not true hallucinations). This is most likely the origin of the mescal cactus's name.