Sideways Surge update 1.4 is out now, and the Mythic loot pool has expanded to include multiple previously not obtainable Mythics.
Sideways Brutes may now also drop the following Mythics:
Striker Burst Rifle
Striker Pump Shotgun
Combat SMG
Heavy Sniper Rifle
Just like before this update, they can also drop the Assault Rifle, Burst Assault Rifle, Minigun and Pulse Rifle. Jump in and kick some monster butt!