Right. Cower in fear at this marvelous (heh, geddit, cuz Marvel Sca--*shot*) ahem, at this marvelous beast, terrible and yet beautiful, regal as the Queen of the Sea, Mega Milotic!
It's only eminently sensible for Milotic to get a Fairy type Mega, if Gyarados gets a Dark one. I'm sick of the stereotype that almost all Fairies have to be cute or dollish, though. Look at Xerneas! It's regal and slightly scary, not sickening like Whimsicott or Clefable or what have you. And Mega Gardevoir is enough proof that fairies can look dignified. (Not including ugly things like Granbull and Armoatisse.) And yes, I'm aware that Mega Milotic looks like a Dragon typing would fit, and I'm also aware that it gets Dragon type moves and no significant Fairy ones, but I don't see it as a draconic being, even were it not to be a counterpart to Gyarados.
So, biology: Mega Milotic is royal, with wavy veil-like fins and a pointed crest. It develops elongated wing-like fins, much like Gyarados, along its flanks, which are used both as display and when manoeuvring underwater. The scales that cover its body enlarge irregularly and envelop its entire torso, up until the neck (though it's difficult to judge where the neck ends and the body begins for a serpentine Pokemon...) and they acquire a hard, opaque texture, as if they were cut of smooth stone. That's because, as a natural progression from Marvel Scale, Mega Milotic has the ability Multiscale. Fear it.
Base stats: 95/60/79/100/125/81
MEGA stats: 95/70/109/130/155/81
Mirrors (heh, geddit, because Mirror Coa--*shot again*) ahem, it mirrors Mega Gyarados's stats, just as the base forms mirror each other's. But that bulk, with Multiscale, an eminently excellent typing in Water/Fairy, AND Access to Recover? Oh god.
It gets the most annoying move of all time in Scald, and it makes good (or evil) use of it. It also gets Moonblast, as it should have in the first place. And it can make an exceptional defensive use of Rest, Sleep Talk and dual STAB, or even use Dragon Tail for shuffling. And you thought Toxic would get rid of it.
It's not unbreakable, though. Things like Kyurem-B and fast electric or poison types do a number on it, and Ferrothorn and Amoonguss and Celebi, among other Grass types, are its bane unless it runs HP Fire, which only does significant damage to Ferrothorn and comes at the cost of valuable coverage. Multiscale is also broken easily by hazards, weather, or prior damage, especially since Milotic is not that fast and can't hold Leftovers, and it also hasn't any ways to boost its speed. It also hasn't got access to any boosting moves per se, unlike Gyarados, and I feel bad for it, so I'm being sadistic and giving it Calm Mind. Yeah. Now suffer.
It still won't break OU, however. It's not impossible to get past, and it's checkable by various things depending on its moves. Taunt is also a major impediment to the defensive sets.
And there it is. Until later!