"Hagrid has never sent men into our hollow before."
-- Aragog (CS15)
Hagrid takes his Acromantula Aragog to live in a hollow in the Forbidden Forest after being accused of opening the Chamber of Secrets. Aragog lives in “the middle of the misty domed web” with his wife Mosag and hundreds of their descendants (CS15). The hollow is located deep inside the forest; Harry and Ron spend “at least half an hour” following the spiders before being grabbed by the Acromantulas and taken to the hollow (CS15). During the Battle of Hogwarts, Voldemort used the Acromantula’s lair in the forest as his base of operations. The Death Eaters drove out the Acromantulas and conscripted them to fight for the Dark Lord. It was into this clearing, still with webs draped overhead, that Harry walked to sacrifice himself (DH34, DH36).
The story of Hagrid taking something monstrous to live deep in the forest is similar to his hiding Grawp away in another clearing fifty years later. Rowling did say in an interview that Fluffy ended up in the Forest, before eventually being repatriated to Greece. What other odd creatures has Hagrid found hollows or clearings for over the years?