Bestiarum

The Beastarium is apparently on a disk that comes with the Legendary Edition and Collector's Edition of Halo 3. This disk will have models and such of the entities of the Halo Universe. Not much is known about it yet, as it has not been released yet, but supposedly it will have 3D renderings of almost all things Halo 3 related.

The Beastarium is in a book for the Collectors Edition of Halo 3, rather than a disk.