Re-Skinned Scattershot, Located in Mission 1: Osiris
This particular Halo 5 special weapon is actually really easy to find (quite possibly the easiest). Play through Osiris as you normally would and when you get up to the cave-like area where the Kraken first appears, simply walk towards it. Near what appears to be a small crashed vehicle is a ledge. Look over the ledge to see the Loathsome Thing sitting there just waiting to be collected.