Cactuars are a species of antagonistic mobile cactus people found in every mainstream Final Fantasy since VI. Due to less than consistent translations of the series, cactuars are known as "cactrots" in Final Fantasy VI and "sabotenders" in Japanese. Cactuars add much needed humor and character to the battle system and monsters of the series.
These little bundles of needley joy are about a meter tall, but thin enough that both hands could probably wrap around their green, cactus bodies. A cactuar has four limbs connected to a pencil shaped body, each pointing in a different direction at all times. Both arms and legs make Tetris like L shapes, but always in opposing directions. At the top of their body is a relatively featureless face- Two small black holes for a pair of eyes, and a perpetually surprised "O" for a mouth. Two to three tiny hairs grow off their heads, drawing the only possible similarity they could have to Homer Simpson.
Square's graphic department spared every expense in animating these fellows. In battle, they only have two frames of animation that alternate the directions of the arms and legs while giving them a jerky, self-consciously fake motion. The obvious comparison to the perpetual two frames of motion Dragon Warrior characters from the NES installments exhibited.
The only common place to find Cactuars is in the desert, considering they are some demented form of cacti. In FFVIII, they have a whole island to themselves known as Cactuar Island.
In battle, cactuars are usually frustrating to deal with. After a random encounter ensues, the screen loads, and cactuars appear on the screen, they usually run away before any action can be taken. If by chance they don't escape, they are difficult to hit as their skill in evasion is about as high as it can go. Sometimes, when a cactuar sticks around long enough, he'll shoot 1000 needles, each of which doing 1 damage for a total of 1000 damage. This is quite an annoyance, considering it's coming from such a small, elusive creature.
Knowing all this, you might wonder why even bother to fight a cactuar if they're nothing but a dangerous hassle. The answer is that cactuars have ridiculously low hit points (only 3 or less in most of the games) and if by some chance you are able to hit them, they are defeated instantly. And victory over cactuars brings various great rewards depending in which Final Fantasy they are defeated. Once again drawing a comparison to the Dragon Warrior series, they could be compared to the metal slimes and babbles, but with more of an offensive threat.
There are many instances where cactuars have particularly special roles. In Final Fantasy VIII, there is an enormous cactuar known as the Jumbo Cactuar. All the humor and deliberate graphic flaws are magnified ten times by the gigantic cactuar jerking back and forth throughout the battle. The Jumbo Cactuar also has a really goofy moustache over its ovular mouth. Defeating Jumbo allows the player to take him as a guardian force.
In Final Fantasy X, there is a sub species of cactuar known as qactuar. They are weaker, but share all the same characteristics. Also, there is the infamous Cactuar Village side quest, complete with hours of wandering a desert in search for the goofy little creates and a handful of "Sphere del Perdedors" to go along. The most amazing cactuar of all, however, is the Cactuar King. Being dubbed "king" gives him the power to use an unblockable, unsurvivable attack called 99,999 needles. Following the thirty second long animation of the king firing volley after volley of needles, it basically does 99,999 damage to a character without exception.
Sources:
Playing the games
http://tonberry-cactuar.tripod.com/