Blanket octopuses, an octopus that lives in the surface to mid-water areas of subtropical and tropical oceans, have membranes on their tentacles that act like cloaks, making them appear larger and thus scaring off potential predators. Males are especially in need of their giant mesh-like membranes, as the octopus exhibits clear sexual dimorphism - males are only a few centimeters long, while females can grow to an astonishing 2 meters.
When the males unfold their reticular membranes to no avail, some will choose to eject ink and escape, but some will take up arms to resist.
As for what kind of weapon it is? Those are the tentacles of the man-of-war jellyfish!
Man-of-war jellyfish are one of the most venomous animals in the ocean. They are as poisonous as cobras on land, and their toxins are stored in the stinging cells of their tentacles.
Because the firing of stinging cells is a completely conditioned reflex, the tentacles remain aggressive even if the jellyfish itself is dead.
Blanket octopuses are immune to the venom of man-of-war jellyfish, and they know very well that the venom of man-of-war jellyfish is very deadly to other animals, so they kill the jellyfish and rip off their tentacles to decorate themselves.
Males and immature females do this, but once females are old enough, they stop using this strategy.