Suicune is my favorite of the legendary dogs, so I'm starting with it and will do the other two. Suicune gains the fairy type upon mega evolving due to its pure aura. Its ability (another made up one, sorry but this would be so useful) would be Purify, getting rid of all hazards on your side when it mega evolves or switches in. It also prevents your team from getting poisoned while its on the field. Suicune gets major boosts to its defensive stats, primarily its sp defense. It also gets smaller boosts to its speed and sp attack. The crest on its head gets bigger and has traces of pink on the edges. Its ribbons turn light pink.the diamonds on its fur turn light pink too. The fur on its back has blue stripes in it and it has pink coils on its legs.
Stat boosts:
45 sp defense
35 defense
15 sp attack
5 speed

This is my 30th Mega Evolution! Thank you all for your votes and comments! Since it's my 30th, I'm going to let you decide what legendary dog should be next. As soon as either entai or rakiou get 3 votes I'm doing that one.