Cactuars are recurring creatures in the Final Fantasy series along with Chocobos and Moogles.
Cactuars are distinguished as little green cacti-men with arms and legs and some form of unusual expression on their faces. They generally don't stay still, moving their arms and legs when the characters are in battle with one. They first appeared in Final Fantasy VI (called Final Fantasy III when released in the US).
Cactuars are generally "hit-and-run" enemies. They will sometimes attack unprovoked, though it usually takes an attack on one of them to retaliate with their signature move, "1,000 Needles," which does exactly 1,000 damage, regardless of the target's defense or stats. This attack is usually followed by a Flee move. Cactuars never stay in a battle for long, and even if they don't perform 1,000 Needles, they will flee.
Some Final Fantasy games have featured tweaks on the Cactuar concept. For example, Final Fantasy VIII features a Jumbo Cactuar, which is both an optional boss and optional GF (summon). It has the move called "10,000 Needles," which is the upgraded "1,000 Needles." It does exactly 10,000 damage (basically a guaranteed kill on any party member) regardless of defense or stats.
Final Fantasy X-2 also added to the Cactuar profile, saying that the Cactuars were actually children Cacti. There is a mini-game where you help the Cacti from the Cactuar Nation find 10 missing Cactuar children. They are all a part of completing the Episode. You must fight and defeat a Jumbo Cactuar (much like in Final Fantasy VIII), and it's also an Oversoulable monster.
Final Fantasy XI featured the first major design change of the Cactuar since it's introduction in FFVI. It was designed to appear somewhat more lifelike. Also some appear with their Japanese name "Sabotender." Maintained still though were the cactus's signature high evasion, lightning quick attacks, and the fear of the "1,000 Needles" attack.
Cactuar now appears in Mario Hoops 3 on 3 as an unlockable character. Its signature shot is 1000 Needles.