Two-headed Trolls
Lore
Two-headed Trolls are a rare and terrifying breed of Troll-kind. They are much larger than ordinary Trolls, possessing a towering height that rivals that of many Giants. Their skin is a stony grey hue and they have wide, flat feet like those of an Oliphaunt which they use to stomp their prey into submission. Their most unsettling feature though is their pair of heads, each of which often has its own name and personality. It's unclear whether the Two-Headed Trolls are malformed freaks of nature or an intentional creation of the Dark Lord Sauron. Whatever the case, they have proven to be very real and dangerous foes.
One such creature is the frightful Gast-Nûl, overseer of the Glimmerdeep mine in Ered Mithrin. He was (or they were) once thought to be a solitary monster by adventurers. However, the existence of a species of Two-headed Trolls was confirmed during the War of Three Peaks when the beasts were spotted among the forces of Mount Gundabad. One of the most dangerous so far reported is Narg-and-Mauz who dominates the deeper reaches of Bagûd-mekhem.
Reported sightings:
While they are rarely seen beyond their homes in the northern mountain ranges, the stories of Two-headed Trolls have spread far and wide. The Troll breed known as Ettens, for example, wear cloaks of animal-skins that give them the appearance of having two heads, an attempt on their part to evoke the legends.
Background
J.R.R. Tolkien briefly mentions two-headed trolls in The Hobbit, though they are not depicted in the story. It is unclear whether such creatures truly existed in Middle-earth or if they were simply legendary.