This is a compilation of Mega Gyarados' pokedex entries from various Pokemon Games.

Sun

A tale is told of a town that angered Gyarados. Before the sun rose the next day, flames utterly consumed the town, leaving not a trace behind.

Moon

There are people who swear that any place Gyarados appears is fated for destruction.

Ultra Sun

It fires hyper beams in all directions, burning the surrounding area to ash. There are some regions where it's called "the deity of destruction."

Ultra Moon

The energy from evolution stimulated its brain cells strongly, causing it to become very ferocious.

Let's Go, Pikachu!

Brutally vicious and enormously destructive. Known for totally destroying cities in ancient times.

Sun

Mega Evolution also affects its brain, leaving no other function except its destructive instinct to burn everything to cinders.

Moon

It jets water from the orifices on its sides, streaking above the water surface at supersonic speed.

Ultra Sun

Mega Evolution places a burden on its body. The stress causes it to become all the more ferocious.

Ultra Moon

It zooms out of the water at Mach speeds. Even large ships caught in its path are split cleanly in two!

Let's Go, Pikachu!

Although it obeys its instinctive drive to destroy everything within its reach, it will respond to orders from a Trainer it truly trusts.