Blue Jackal
From Halopedia, the Halo wiki
On Metropolis, kill a Jackal on the Scarab. If it is on the right place, it will appear blue, as though it were inside the Scarab. This is due to error in ground establishment on the Scarab (a part of the Jackal is "inside" the Scarab, and anything labeled as "inside" assumes a blue tinge). Not sure, but may work with other species.
