From Halopedia, the Halo wiki
The Bestiarum is a Forerunner-designed compendium of information about the various species in the Milky Way Galaxy. Compiled by a Forerunner archival catalog, the Bestiarum also includes information about the species' homeworlds and the Forerunners' Technological Achievement Tiers. Sources for the information include the Halo Array, Shield Worlds, 2401 Penitent Tangent, and 343 Guilty Spark, with the latter Monitor's observations "under investigation".
Bestiarum - (CAVEAT)
The following information has been gleaned from a variety of sources, impartial and otherwise. Records retrieved from alien systems cannot be verified except by cross-reference against our own records. The significant system-wide lull in record-keeping for many of these species invalidates much of the provable veracity of this information. The following document is as accurate as possible under difficult circumstances but should be treated as an incomplete investigation only.
Investigation Status: ONGOING.
Information origins include:
UNSECURED ALIEN COMPUTER ARCHIVES (SUSPECT)
DNA/RNA/Silicon samples are encoded in this datastream. Reproduction of individuals for analysis is prohibited by this facility.
Tiers of Technology