Mountain-trolls

Middle-earth contained various different kinds of Trolls adapted to living in different environments, and among these kinds was a variety known as Mountain-trolls. We know little about these Trolls or where they originated, but at the end of the Third Age, at least some were in the service of Sauron. In the Battle of the Pelennor Fields, when the great battering ram named Grond was deployed to break down the Great Gate of Minas Tirith, it was Mountain-trolls who were tasked to wield it. They must therefore have been among the strongest of Sauron's servants.

We know that the variety of Trolls known as Stone-trolls could not survive in sunlight, which would literally turn them to stone, but it is unknown whether this same curse afflicted Mountain-trolls. Their work with Grond was completed during the night, so this is inconclusive, but the Great Gate did not break until dawn was approaching. So, if Mountain-trolls did indeed turn to stone in sunlight, they must have hurried into shelter immediately after completing their task.

A variety of Troll known as a 'Hill-troll' was common in various places across Middle-earth, including Mordor, and that name seems very close to 'Mountain-troll'. Based on the names (which is essentially the only evidence we have) we might expect that Hill-trolls and Mountain-trolls were related kinds, or perhaps even different names for the same Troll variety. Hill-trolls were known to be large with scaled skin, so it might be reasonably imagined that Mountain-trolls were at least somewhat similar.