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.
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.
Easiest take ever surprisingly. Walking through redwoods and thought I saw a bear so went to feed my carno and found one hung between rocks. It couldn’t move so I shot and tamed right there. Only a level 3 but used no real effort and first one I’ve seen
Completely docile unless you attack it. Already bred a high pair got a 245 baby. Took a long time to fully mature though. About the same as a Rex. When running away go between rocks as it is surprisingly fast.