In my last post, I admitted that I could not speak for 5e, due to my relative unfamiliarity with it, but made that case that monsters in MM1 are implicitly and explicity personifications of something seriously wrong with the world. Out of curiosity, I decided to look up the monster descriptions from several of the monsters I used as examples from the MM1 in the 5e MM. This is what I found:
Blink dogs harbor a long-standing hatred for displacer beasts and attack them on sight.
Some sages believe the bulette is the result of a mad wizard's experiments at crossbreeding snapping turtles and armadillos, with infusions of demon ichor.
The warriors of [the Unseelie Court] selectively bred the beasts to reinforce their ferocious and predatory nature, using them to hunt unicorns, pegasi, and other wondrous prey.
The origin of the gnolls traces back to a time when the demon lord Yeenoghu found his way to the Material Plane and ran amok.
Across the borderlands of civilization, settlements and settlers must contend with these aggressive humanoids, whose thirst for conquest is never satisfied.
Grasping his mighty spear, he laid waste to the mountains, set the forests aflame, and carved great furrows in the fields. Such was the role of the orcs, he proclaimed, to take and destroy all that the other races would deny them. To this day, the orcs wage an endless war on humans, elves, dwarves, and other folk.
The most common theory is that a demented wizard created the first specimen by crossing a giant owl with a bear.
So, 5e also knows how to do monsters, too.