Cacti is the Latin plural of cactus, and some writers use it in English. Cactuses is the English plural. Dictionaries list both, and neither is right or wrong. Also, like many names of plants, the uninflected cactus is sometimes treated as plural.
The prevalence of the Latin cacti can be attributed to the influence of Latin on biological nomenclature. These Latin plurals are not considered out of place in botany and other scientific fields, and some make their way into broader usage, but there’s no good reason that the ordinary English speaker should have to abide by the rules of Latin grammar.
Cactus is not the only Latin-derived English word ending in –us, and most are conventionally pluralized in the English manner. Fungus, like cactus, often becomes fungi (though funguses is just as good), but this is one of the few exceptions. Most English speakers don’t say ani instead of anuses, apparati instead of apparatuses, campi instead of campuses, octopi instead of octopuses, stati instead of statuses, or viri instead viruses, and there’s no reason cactus should be any didfferent. It’s a matter of preference, though, and cacti is not wrong.
Though cacti has the edge, both forms are common in current news publications and blogs from throughout the English-speaking world—for example:
For days, he trekked through desert plains dotted with cacti as vultures circled above his head. [Newsday]
Text-book cactuses flank the picturesque 449-yard, par-four 14th. [BBC]
Ringed by cacti and red rock buttes and canyons, the town looks like it once was the set of every shoot ’em up cowboy movie. [National Post]
But near the visitor center, which was dug into the earth, irrigated areas coaxed succulent blooms from cactuses. [Minneapolis Star Tribune]
One room has been converted into a type of conservatory, with tiny monkeys swinging from plants, cacti growing out of an old ball-and-claw couch and succulents everywhere. [Mail and Guardian]
All cacti are succulents, but many other plant families include succulent species. [Monterey County Herald]