Barrow-Wights

Barrow-wights themselves are a specific type of undead creature in Middle-earth, they are wraith-like beings animated by evil spirits, often guarding tombs or barrows. They are found in places like the Barrow-downs in Eriador and are depicted as ancient kings or nobles whose spirits have been twisted to malevolence. Barrow-wights are distinct from other major evil species but related as types of undead or spirit-animated beings.

To clarify, Barrow-wights are not a species group with subspecies like "balrogs" (balrogs are not barrow wights, they are maiar) Instead, they are a category of undead creatures. However, related or similarly "undead" or wraith-like entities in Arda.