Everybody knows man was the real monster all along. We can't do tail-whips though, so we're not nearly as fun to fight. Modder "UberGrainy" has instead plopped the xenomorph queen from Aliens into Monster Hunter: World, letting you go toe to toe with the double-mouthed mother of fear.
You download the mod here. I think my favourite part of this is imagining H.R. Giger's reaction. There he goes, crafting the apex of menace, an unstoppable killing-machine that's smarter and more sinister than anything humanity has ever imagined. There modders go, letting you beat it up with swords.
Here's a video of it being stabbed by 2B from Nier: Automata.
That Nier skin is also courtesy of "UberGrainy", which you grab here.
Sadly, there seems to be a real paucity of mods that add in new monsters. My search turned up nothing - other than a couple of "UberGrainy's" other mods, which pit you against Mufasa and Thomas the Tank Engine. Solid choices.
As is this pig.
You'll have to figure out how to best those beasties by yourself, but guidesman Dave has covered literally anything else you could ever want to know in his Monster Hunter guides.