Olog-hai were an elite stock of trolls that were likely bred by Sauron for the purposes of serving him as elite shock troops in his armies. They were a hardier form of troll than Cave Trolls, and were not turned to stone by daylight.
It is not entirely clear when or how the Olog-hai first came into being, but presumably they were created by Sauron in some way (possibly by infusing pre-existing Trolls with enough his malice or power to make them stronger). In any event, come the War of the Ring, the Olog-hai were used by him extensively in his campaigns against Gondor and elsewhere (such as in Mirkwood). This is especially true in the movies, where they fill a variety of specific functions, including drummers and pushing siege towers into position. Ones who engaged in up-close fighting and smashing aside defenses and enemy soldiers were known as "Attack Trolls". One such Attack Troll engages Aragorn at the Black Gates near the end of Return of the King. In the books, it is Pippin who fights and kills a Troll, though it is unclear if it was an Olog-hai or not.
- Just as Uruk-hai actually just translates to "Orc-Race", so too does "Olog-hai" mean "Troll Race".
- As a superior off-shoot of the original race that is tougher, stronger, and not as negatively affected by sunlight, Olog-hai are essentially to lesser trolls what Uruk-hai are to Orcs.
- In Middle-Earth: Shadow of War, numerous Olog-hai are encountered, all of them significantly more intelligent and capable of reason than they are shown to be in the books and movies. They can essentially have all of the same roles as Uruks, up to and including War-Chief.