Cactus is one of the 20 cards added in Beta 6.6. Previous Versions can be found here.
This is a decent defensive card. It can kill any monster damaged by this one, no matter how much HP they have. Of course, this excludes cards that don't get damaged at all in a certain situation, such as Mad Dummy, Dodge monsters like Sans or Armor monsters like Charles.
Since Beta 51.0, it has 1 ATK and no Disarmed, meaning it can make use of its effect without the need of another card. However, it only has 3 HP, making it very frail.
It can somewhat be seen as a detterend to Mace, being something that can easily die in 1 hit but will usually take its attacker with it. But with its lack of Taunt, the opponent can simply ignore this (at the cost of them risking one of their monsters dying next turn).