Like many other creatures, the buff of wild Megatherium is applied on sight of the target (all kind of arthropods) rather than after performing the action needed to activate it once tamed (killing), so don't expect to tackle one while there are any tamed or wild arthropods nearby, including even the strongest of the Mantis trained in melee.
It is also worth note that wild Megatherium will become aggressive when the buff is activated, so stay clear of them unless you have means to take it down.
THIS MONSTER LITERALLY JUST KILLED 3 OF MY(and my bf's) BEELZEBUFOS, 1 ANKY AND 1 ARCHY. I nagged my bf for so long to tame the beelzebufos and felt so connected to those cute froggies. RIP Winky, Pepe, Froppy, Kyle and Argentina #neverforget 😭