Since Gyarados got a Mega evolution, it's only fair for Milotic to get one. Mega Milotic would gain the dragon type and get the ability magic guard, making you have to attack it directly. Dragon would give its powerful egg move dragon pulse STAB, as well as eliminate its weaknesses to grass and electric. Its stat boosts would include a defensive boost as well as a sp attack boost. Mega Milotics appearance would be a bigger tail and a blue-red pattern down its entire body as well as dark red frill down its back.
Stat boosts:
30 defense
30 sp def
10 speed
30 sp attack