The Queen Big Snail is a monstrous huge snail, exclusive to Metal Slug Attack. It first made its debut on the game's event titled 'Halloween Forest', where the boss can also become a playable unit. .
"A snail that has grown to an enormous size in a subterranean world unexplored by mankind. It hides its huge body in the ground when it cannot longer fit into its shell. It preys on objects touched by the tips of their long tentacles randomly, so eyewitness reports are rare."
- ~Metal Slug Attack in-game description.
In Metal Slug Attack's event, this boss is fought at the last stage. It appears underground and slowly moves to attack to the player with his tentacles putting them tout of the ground, only stopping when it encounters one of the player's units near of it, after which it appears and attacks them with its tentacles,but if the player's units comes towards to it, they will be damaged by its poisonous slime.
The special attack of the boss lets prompts it to appear immediately without having to locate an enemy unit. Upon appearing, Queen Big Snail unleashes its tentacles as whips, lashing out at enemy targets; also, its tentacles can target the player's units no matter how close they are.. Queen Big Snail can also avoid incoming attacks by submerging back underground if is there no player's units near to it, so players are required to attack it to so much make it easier to hit.