Gyarados

Gyarados is a pokemon from Generation I. The evolved form of Magikarp, this water/flying type pokemon is known as the atrocious pokemon. It evolves from Magikarp at Level 20, and then mega evolves with Gyaradosite into what is known as the dark/water type Mega Gyarados. Many people have owned a Gyarados in the anime including, James (for a short time), Misty (for only a Pokemon Inspection Agency Test), and the infamous red Gyarados (forcibly evolved from a Magikarp to Gyarados extrafast).

Competitively, Gyarados is great, get an adament one with  intimidate and leftovers, and train it in speed and attack. For his first move you could either use substitute or taunt; (both work great) dragon dance will work good here too boosting his stats even more, and as for the last two moves use waterfall and bounce. But! if you want to use Gyaradosite here's another option, replace waterfall with dragon tail and bounce with earthquake. Don't forget to get an impish one!

Trivia:

Gyarados is a large dragon Pokémon most similar in appearance to those seen in Chinese mythology. The concept of Magikarp evolving into Gyarados is based on the ancient Chinese tradition that the carp would evolve into a dragon after swimming upstream.