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.
Great chitin farmer.
Be careful around these monsters, one of them destroyed my MC armor in 2 hits and killed me instantly, build a pen or bring an animal to tank it while you tranquilize the f*er, good luck survivor!
If you're in the snow biome you shouldn't worry about them too much. But If you're in the redwoods keep track of where these guys are at all times cause any second the game could spawn ants or dragonflies out of nowhere and if you are anywhere close to thi guy kiss your ass goodbye. I speak from experience, i lost a pack of hyenadon i had just tamed to this guy.
To cheese set down stone pillar and let it kill you return and it will only attack pillar doing no damage the just shoot it till it dies (don't let it kill you too close to the pillar or you won't be able to retrieve your stuff without it attacking you, about 10ft away should be safe)