The origin of the word cactus dates to 1600, when it was used to describe a prickly plant of Sicily, the Spanish artichoke (known as a cardoon or artichoke thistle). Although the artichoke is prickly, it is not similar in appearance to the cactus we typically envision in America. The use of the word cactus in English began in 1769 to describe the green, leafless plants of the American desert. The most common cactus that we think of in the U.S. is the saguaro cactus of the Southwest. Cactus has a Latin root that derives from the plural cacti, which makes cacti the technically proper plural spelling. If you want to stay on the right side of the rulebook, you can refer to more than one cactus as cacti. At the same time, as cactus was assimilated into English, it inevitably received English treatment, which often pluralizes a word by adding s or es. As a result, cactuses became an accepted plural as well.

Source: Grammar Book