I saw a Magikarp using Hydro Pump in this video:- https://www.youtube.com/watch?v=pYElZonHQ0I
Is it true or is the Karp hacked ?
That Magikarp is actually an Event Pokemon.
Event Pokemon are special Pokemon given out over the Nintendo Network or at select locations during a small time frame.To get these Pokemon, you must choose "Mystery Gift" at the main menu of your Pokemon Omega Ruby or Alpha Sapphire game. The next step depends on the method of distribution. Some Pokemon are distributed over the internet, some are at select areas via wireless connection, and some are obtained through special codes.
In this case, it was a 2013 Magikarp Event exclusive to Japan that comes with these special features. Source Source
The Magikarp in the video is hacked. The only moves a Magikarp can have without hacking are Splash, Tackle, Flail, Bounce, Celebrate, and Happy Hour.
Source: http://www.serebii.net/pokedex-xy/129.shtml
Hope I helped!
