Using the set
AM posted, Mega Latios ca set-up on on the following.
Keldeo:
252 SpA Choice Specs Keldeo Hydro Pump vs. 16 HP / 0 SpD Mega Latios: 114-135 (37.3 - 44.2%) -- guaranteed 3HKO
Char-Y:
252 SpA Mega Charizard Y Fire Blast vs. 16 HP / 0 SpD Mega Latios in Sun: 134-158 (43.9 - 51.8%) -- 11.7% chance to 2HKO
Mega Camerupt:
252+ SpA Sheer Force Camerupt Fire Blast vs. 16 HP / 0 SpD Mega Latios: 118-140 (38.6 - 45.9%) -- guaranteed 3HKO
Mega Venu:
252+ SpA Mega Venusaur Sludge Bomb vs. 16 HP / 0 SpD Mega Latios: 133-157 (43.6 - 51.4%) -- 6.6% chance to 2HKO
Mega Manectric:
252 SpA Mega Manectric Hidden Power Ice vs. 16 HP / 0 SpD Mega Latios: 116-138 (38 - 45.2%) -- guaranteed 3HKO
Heatran (worst case scenario, HP Ice ScarfTran):
252+ SpA Heatran Hidden Power Ice vs. 16 HP / 0 SpD Mega Latios: 124-146 (40.6 - 47.8%) -- guaranteed 3HKO
Landorus-I (weaker set-up, you can't come in on Knock Off and must have Ice Beam):
0 Atk Life Orb Landorus Knock Off vs. 16 HP / 0- Def Mega Latios: 164-195 (53.7 - 63.9%) -- guaranteed 2HKO
Those are the common ones, there are others of course.
theV8man Rotom really isn't an option because of WoW, but yeah everything else Latios checks you can set-up on.
AM I initially did some calcs with a Naive and found a Hasty nature actually gives it more set-up opportunities, especially if you run the first four calcs with Hasty instead of Naive.