Source: Monster Manual 5e
The Mighty Dragon

Within the depths of snowy mountains lay many caverns, claimed not by the Dwarves but by the Dragons!

Source: Monster Manual 5e
The Versatile Beholder

From disintegeration rays, to beams that force a deep sleep, to the infamous Antimagic Gaze of it's largest eye, the Beholder's are of some of the most versatile and dangerous creatures you may encounter.

One must stay aware of their surroundings and remember that where the Beholder's gaze sets, death follows...

Source: Monster Manual 5e
The Malignant Illithid

The Illithid is a malignancy, gradually poisoning the realm. Their origins are mysterious and their enemies are many.

Source: Monster Manual 5e
The Treacherous Mimic

A species of peculiarities. Mimics are creatures of intense shapeshifting talent, able to pass themselves off as any mundane object with ease.

And remember... never let the rogue go off alone, lest they be caught in it's maw.