These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Fantastic Beasts and Where to Find Them
Nightmare Fuel: The Nundu, which resembles a Leopard — said to be utterly silent, gigantic, and able to wipe out entire villages just by breathing due to the diseases on its breath; it's so dangerous that the only successful kills have come from one hundred wizards working in concert. For comparison, that's ten times as many wizards as it takes to subdue the average dragon.
The Lethifold, mentioned above, would also count, due to the fact that they sneak up on unsuspecting victims and smother them in their sleep. And digests them on the spot and leaves no evidence of their demise.
The Manticore, a Shout-Out to classical Greek Mythology. It has a man's head, a lion's body, and a scorpion's tail, and its sting causes instant death. If that wasn't enough, its skin is nigh-invulnerable to magic. Not to mention it can speak intelligently and croons while it eats its prey. And Hagrid possibly crossbred one.note It's a Rita Skeeter article, so take it with a grain of salt, but if anyone would crossbreed manticores with fire-crabs, it'd be Hagrid.
Not to mention the only record of a Chimera being killed "led to the unlucky wizard in question falling from his winged horse".note In the original legend, after slaying it while riding the winged horse Pegasus, Bellerophon tried to fly to Mount Olympus (where the gods lived). Guess how well that turned out for him.
Then there's the Quintapeds. According to legend they were once a family of wizards before they were transformed into monsters by a rival wizard family, who were in turn eaten by the beasts they created. Whether or not the legend is true, these five-legged carnivorous beasts rate five Xs, and are so dangerous the island they live on has been marked completely hazardous and enchanted so it can't be placed on maps. God help us all if those things are capable of breeding.