Snow-trolls

Apparently, a kind of Troll that was adapted for life in conditions of snow and ice. In fact, no Snow-troll appears directly in Tolkien's work, but King Helm Hammerhand was compared to such a creature as he made sorties against the Dunlendings during the Long Winter. The King is specifically said to have been clad in white during these forays, which can perhaps be taken to imply that Snow-trolls were white in colour (presumably this means that they were covered in white fur, though with so little detail to assess, even this is ambiguous).

It should be said that it is not even certain that Snow-trolls existed at all. We have only a single metaphorical reference, and even this might be taken as simply comparing Helm's ferocity to that of a Troll in the snow. On the other hand, there were known to have been different kinds of Trolls in Middle-earth (such as Cave-trolls or Mountain-trolls) so the existence of Snow-trolls seems entirely plausible. On balance, the more natural reading seems to be that 'Snow-trolls' were real (and evidently dangerous) creatures in Middle-earth.